Upstream oodles of Chrome for Android code into Chromium.

chrome/android/java_staging/src/org/chromium/chrome/browser/compositor/layouts/phone/stack/Stack.java

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package org.chromium.chrome.browser.compositor.layouts.phone.stack;
+
+import android.animation.Animator;
+import android.animation.AnimatorListenerAdapter;
+import android.animation.AnimatorSet;
+import android.content.Context;
+import android.content.res.Resources;
+import android.graphics.RectF;
+import android.view.animation.AccelerateDecelerateInterpolator;
+import android.view.animation.Interpolator;
+
+import com.google.android.apps.chrome.R;
+
+import org.chromium.base.annotations.SuppressFBWarnings;
+import org.chromium.base.metrics.RecordUserAction;
+import org.chromium.chrome.browser.Tab;
+import org.chromium.chrome.browser.compositor.layouts.ChromeAnimation;
+import org.chromium.chrome.browser.compositor.layouts.Layout;
+import org.chromium.chrome.browser.compositor.layouts.Layout.Orientation;
+import org.chromium.chrome.browser.compositor.layouts.components.LayoutTab;
+import org.chromium.chrome.browser.compositor.layouts.eventfilter.EdgeSwipeEventFilter.ScrollDirection;
+import org.chromium.chrome.browser.compositor.layouts.phone.StackLayout;
+import org.chromium.chrome.browser.compositor.layouts.phone.stack.StackAnimation.OverviewAnimationType;
+import org.chromium.chrome.browser.tabmodel.TabModel;
+import org.chromium.chrome.browser.tabmodel.TabModelUtils;
+import org.chromium.chrome.browser.util.MathUtils;
+import org.chromium.ui.base.LocalizationUtils;
+
+/**
+ * Handles all the drawing and events of a stack of stackTabs.
+ *
+ * @VisibleForTesting
+ */
+public class Stack {
+    public static final int MAX_NUMBER_OF_STACKED_TABS_TOP = 3;
+    public static final int MAX_NUMBER_OF_STACKED_TABS_BOTTOM = 3;
+
+    private static final float STACK_PORTRAIT_Y_OFFSET_PROPORTION = -0.8f;
+    private static final float STACK_LANDSCAPE_START_OFFSET_PROPORTION = -0.7f;
+    private static final float STACK_LANDSCAPE_Y_OFFSET_PROPORTION = -0.5f;
+
+    public enum DragLock { NONE, SCROLL, DISCARD }
+
+    /**
+     * The percentage of the screen that defines the spacing between tabs by default (no pinch).
+     */
+    public static final float SPACING_SCREEN = 0.26f;
+
+    /**
+     * The percentage of the screen to cover for the discarded tab to be fully transparent.
+     */
+    public static final float DISCARD_RANGE_SCREEN = 0.7f;
+
+    /**
+     * The percentage the tab need to be dragged to actually discard the card.
+     */
+    private static final float DISCARD_COMMIT_THRESHOLD = 0.4f;
+
+    /**
+     * The percentage of the side of the tab that is inactive to swipe to discard. As this is
+     * a distance computed from both edges, meaningful value ranges in [0 ... 0.5].
+     */
+    private static final float DISCARD_SAFE_SELECTION_PCTG = 0.1f;
+
+    /**
+     * The minimum scale the tab can reach when being discarded by a click.
+     */
+    private static final float DISCARD_END_SCALE_CLICK = 0.7f;
+
+    /**
+     * The minimum scale the tab can reach when being discarded by a swipe.
+     */
+    private static final float DISCARD_END_SCALE_SWIPE = 0.5f;
+
+    /**
+     * The delta time applied on the velocity from the fling. This is to compute the kick to help
+     * discarding a card.
+     */
+    private static final float DISCARD_FLING_DT = 1.0f / 45.0f;
+
+    /**
+     * The maximum contribution of the fling. This is in percentage of the range.
+     */
+    private static final float DISCARD_FLING_MAX_CONTRIBUTION = 0.4f;
+
+    /**
+     * How much to scale the max overscroll angle when tabs are tilting backwards.
+     */
+    private static final float BACKWARDS_TILT_SCALE = 0.5f;
+
+    /**
+     * When overscrolling towards the top or left of the screen, what portion of
+     * the overscroll should be devoted to sliding the tabs together. The rest
+     * of the overscroll is used for tilting.
+     */
+    private static final float OVERSCROLL_TOP_SLIDE_PCTG = 0.25f;
+
+    /**
+     * Scale max under/over scroll by this amount when flinging.
+     */
+    private static final float MAX_OVER_FLING_SCALE = 0.5f;
+
+    /**
+     * mMaxUnderScroll is determined by multing mMaxOverScroll with
+     * MAX_UNDER_SCROLL_SCALE
+     */
+    private static final float MAX_UNDER_SCROLL_SCALE = 2.0f;
+
+    /**
+     * Drags that are mostly horizontal (within 30 degrees) signal that
+     * a user is discarding a tab.
+     */
+    private static final float DRAG_ANGLE_THRESHOLD = (float) Math.tan(Math.toRadians(30.0));
+
+    /**
+     * Reset the scroll mode after this number of milliseconds of inactivity or small motions.
+     */
+    private static final long DRAG_TIME_THRESHOLD = 400;
+
+    /**
+     * Minimum motion threshold to lock the scroll mode.
+     */
+    private static final float DRAG_MOTION_THRESHOLD_DP = 1.25f;
+
+    /**
+     * The number of attempt to get the full roll overscroll animation.
+     */
+    private static final int OVERSCROLL_FULL_ROLL_TRIGGER = 5;
+
+    /**
+     * Percentage of the screen to wrap the scroll space.
+     */
+    private static final float SCROLL_WARP_PCTG = 0.4f;
+
+    /**
+     * Percentage of the screen a swipe gesture must traverse before it is allowed to be canceled.
+     */
+    private static final float SWIPE_LANDSCAPE_THRESHOLD = 0.19f;
+
+    /**
+     * How far to place the tab to the left of the user's finger when swiping in dp.  This keeps the
+     * tab under the user's finger.
+     */
+    private static final float LANDSCAPE_SWIPE_DRAG_TAB_OFFSET_DP = 40.f;
+
+    // External References
+    private TabModel mTabModel;
+
+    // True when the stack is still visible for animation but it is going to be empty.
+    private boolean mIsDying;
+
+    // Screen State Variables
+    private int mSpacing;
+    private float mWarpSize;
+    private StackTab[] mStackTabs; // mStackTabs can be null if there are no tabs
+
+    private int mLongPressSelected = -1;
+
+    // During pinch, the finger the closest to the bottom of the stack changes the scrolling
+    // and the other finger locally stretches the spacing between the tabs.
+    private int mPinch0TabIndex = -1;
+    private int mPinch1TabIndex = -1;
+    private float mLastPinch0Offset;
+    private float mLastPinch1Offset;
+
+    // Current progress of the 'even out' phase. This progress as the screen get scrolled.
+    private float mEvenOutProgress = 1.0f;
+    // Rate to even out all the tabs.
+    private float mEvenOutRate = 1.0f; // This will be updated from dimens.xml
+
+    // Overscroll
+    private StackScroller mScroller;
+    private float mOverScrollOffset;
+    private int mOverScrollDerivative;
+    private int mOverScrollCounter;
+    private float mMaxOverScroll; // This will be updated from dimens.xml
+    private float mMaxUnderScroll;
+    private float mMaxOverScrollAngle; // This will be updated from values.xml
+    private float mMaxOverScrollSlide;
+    private final Interpolator mOverScrollAngleInterpolator =
+            new AccelerateDecelerateInterpolator();
+    private final Interpolator mUnderScrollAngleInterpolator =
+            ChromeAnimation.getDecelerateInterpolator();
+    private final Interpolator mOverscrollSlideInterpolator =
+            new AccelerateDecelerateInterpolator();
+
+    // Drag Lock
+    private DragLock mDragLock = DragLock.NONE;
+    private long mLastScrollUpdate = 0;
+    private float mMinScrollMotion = 0;
+
+    // Scrolling Variables
+    private float mScrollTarget = 0;
+    private float mScrollOffset = 0;
+    private float mScrollOffsetForDyingTabs = 0;
+    private float mCurrentScrollDirection = 0.0f;
+    private StackTab mScrollingTab = null;
+
+    // Swipe Variables
+    private float mSwipeUnboundScrollOffset;
+    private float mSwipeBoundedScrollOffset;
+    private boolean mSwipeIsCancelable;
+    private boolean mSwipeCanScroll;
+    private boolean mInSwipe;
+
+    // Discard
+    private StackTab mDiscardingTab;
+
+    // We can't initialize mDiscardDirection here using LocalizationUtils.isRtl() because it will
+    // involve a jni call. Instead, mDiscardDirection will be initialized in Show().
+    private float mDiscardDirection = Float.NaN;
+
+    private float mMinSpacing; // This will be updated from dimens.xml
+
+    private boolean mRecomputePosition = true;
+
+    private int mReferenceOrderIndex = -1;
+
+    // Orientation Variables
+    private int mCurrentMode = Orientation.PORTRAIT;
+
+    // Animation Variables
+    private OverviewAnimationType mOverviewAnimationType = OverviewAnimationType.NONE;
+    private StackAnimation mAnimationFactory;
+    private StackViewAnimation mViewAnimationFactory;
+
+    // Running set of animations applied to tabs.
+    private ChromeAnimation<?> mTabAnimations;
+    private AnimatorSet mViewAnimations;
+
+    // The parent Layout
+    private final StackLayout mLayout;
+
+    // Border values
+    private float mBorderTransparentTop;
+    private float mBorderTransparentSide;
+    // TODO(dtrainor): Expose 9-patch padding from resource manager.
+    private float mBorderTopPadding;
+    private float mBorderLeftPadding;
+
+    private final AnimatorListenerAdapter mViewAnimatorListener = new AnimatorListenerAdapter() {
+        @Override
+        public void onAnimationCancel(Animator animation) {
+            mLayout.requestUpdate();
+        }
+
+        @Override
+        public void onAnimationEnd(Animator animation) {
+            mLayout.requestUpdate();
+        }
+    };
+
+    /**
+     * @param layout The parent layout.
+     */
+    public Stack(Context context, StackLayout layout) {
+        mLayout = layout;
+        contextChanged(context);
+    }
+
+    /**
+     * @param tabmodel The model to attach to this stack.
+     */
+    public void setTabModel(TabModel tabmodel) {
+        mTabModel = tabmodel;
+    }
+
+    /**
+     * @return The {@link StackTab}s currently being rendered by the tab stack.
+     * @VisibleForTesting
+     */
+    @SuppressFBWarnings("EI_EXPOSE_REP")
+    public StackTab[] getTabs() {
+        return mStackTabs;
+    }
+
+    /**
+     * @return The number of tabs in the tab stack.
+     * @VisibleForTesting
+     */
+    public int getCount() {
+        return mStackTabs != null ? mStackTabs.length : 0;
+    }
+
+    /**
+     * @return The number of visible tabs in the tab stack.
+     */
+    public int getVisibleCount() {
+        int visibleCount = 0;
+        if (mStackTabs != null) {
+            for (int i = 0; i < mStackTabs.length; ++i) {
+                if (mStackTabs[i].getLayoutTab().isVisible()) visibleCount++;
+            }
+        }
+        return visibleCount;
+    }
+
+    /*
+     * Main Interaction Methods for the rest of the application
+     *
+     *
+     * These methods are the main entry points for the model to tell the
+     * view that something has changed.  The rest of the application can
+     * alert this class that something in the tab stack has changed or that
+     * the user has decided to enter the tab switcher.
+     *
+     */
+
+    /**
+     * Triggers the closing motions.
+     *
+     * @param time The current time of the app in ms.
+     * @param id The id of the tab that get closed.
+     */
+    public void tabClosingEffect(long time, int id) {
+        if (mStackTabs == null) return;
+
+        // |id| cannot be used to access the particular tab in the model.
+        // The tab is already gone from the model by this point.
+
+        int newIndex = 0;
+        boolean needAnimation = false;
+        for (int i = 0; i < mStackTabs.length; ++i) {
+            if (mStackTabs[i].getId() == id) {
+                // Mark the {@link StackTab} as dying so that when the animation is
+                // finished we can clear it out of the stack. This supports
+                // multiple {@link StackTab} deletions.
+                needAnimation |= !mStackTabs[i].isDying();
+                mStackTabs[i].setDying(true);
+            } else {
+                // Update the {@link StackTab} with a new index here.  This makes sure the
+                // {@link LayoutTab} end up in the proper place.
+                mStackTabs[i].setNewIndex(newIndex++);
+            }
+        }
+
+        if (needAnimation) {
+            mScrollOffsetForDyingTabs = mScrollOffset;
+            mSpacing = computeSpacing(newIndex);
+
+            startAnimation(time, OverviewAnimationType.DISCARD);
+        }
+
+        if (newIndex == 0) {
+            mIsDying = true;
+        }
+    }
+
+    /**
+     * Animates all the tabs closing at once.
+     *
+     * @param time The current time of the app in ms.
+     */
+    public void tabsAllClosingEffect(long time) {
+        boolean needAnimation = false;
+
+        if (mStackTabs != null) {
+            for (int i = 0; i < mStackTabs.length; ++i) {
+                needAnimation |= !mStackTabs[i].isDying();
+                mStackTabs[i].setDying(true);
+            }
+        } else {
+            // This needs to be set to true to handle the case where both the normal and incognito
+            // tabs are being closed.
+            needAnimation = true;
+        }
+
+        if (needAnimation) {
+            mScrollOffsetForDyingTabs = mScrollOffset;
+            mSpacing = computeSpacing(0);
+
+            if (mStackTabs != null) {
+                boolean isRtl =
+                        !((mCurrentMode == Orientation.PORTRAIT) ^ LocalizationUtils.isLayoutRtl());
+                for (int i = 0; i < mStackTabs.length; i++) {
+                    StackTab tab = mStackTabs[i];
+                    tab.setDiscardOriginY(0.f);
+                    tab.setDiscardOriginX(
+                            isRtl ? 0.f : tab.getLayoutTab().getOriginalContentWidth());
+                    tab.setDiscardFromClick(true);
+                }
+            }
+            startAnimation(time, OverviewAnimationType.DISCARD_ALL);
+        }
+
+        mIsDying = true;
+    }
+
+    /**
+     * Animates a new tab opening.
+     *
+     * @param time The current time of the app in ms.
+     * @param id The id of the new tab to animate.
+     */
+    public void tabCreated(long time, int id) {
+        if (!createTabHelper(id)) return;
+
+        mIsDying = false;
+        finishAnimation(time);
+        startAnimation(time, OverviewAnimationType.NEW_TAB_OPENED,
+                TabModelUtils.getTabIndexById(mTabModel, id), TabModel.INVALID_TAB_INDEX, false);
+    }
+
+    /**
+     * Animates the closing of the stack. Focusing on the selected tab.
+     *
+     * @param time The current time of the app in ms.
+     * @param id   The id of the tab to select.
+     */
+    public void tabSelectingEffect(long time, int id) {
+        int index = TabModelUtils.getTabIndexById(mTabModel, id);
+        startAnimation(time, OverviewAnimationType.TAB_FOCUSED, index, -1, false);
+    }
+
+    /**
+     * Called set up the tab stack to the initial state when it is entered.
+     *
+     * @param time The current time of the app in ms.
+     * @param focused Whether or not the stack was focused when entering.
+     */
+    public void stackEntered(long time, boolean focused) {
+        // Don't request new thumbnails until the animation is over. We should
+        // have cached the visible ones already.
+        boolean finishImmediately = !focused;
+        mSpacing = computeSpacing(mStackTabs != null ? mStackTabs.length : 0);
+        resetAllScrollOffset();
+        startAnimation(time, OverviewAnimationType.ENTER_STACK, finishImmediately);
+    }
+
+    /**
+     * @return Whether or not the TabModel represented by this TabStackState should be displayed.
+     */
+    public boolean isDisplayable() {
+        return !mTabModel.isIncognito() || (!mIsDying && mTabModel.getCount() > 0);
+    }
+
+    private float getDefaultDiscardDirection() {
+        return (mCurrentMode == Orientation.LANDSCAPE && LocalizationUtils.isLayoutRtl()) ? -1.0f
+                                                                                          : 1.0f;
+    }
+
+    /**
+     * show is called to set up the initial variables, and must always be called before
+     * displaying the stack.
+     */
+    public void show() {
+        mDiscardDirection = getDefaultDiscardDirection();
+
+        // Reinitialize the roll over counter for each tabswitcher session.
+        mOverScrollCounter = 0;
+
+        // TODO: Recreating the stack {@link StackTab} here might be overkill.  Will these
+        // already exist in the cache?  Check to make sure it makes sense.
+        createStackTabs(false);
+    }
+
+    /*
+     * Animation Start and Finish Methods
+     *
+     * This method kicks off animations by using the
+     * TabSwitcherAnimationFactory to create an AnimatorSet.
+     */
+
+    /**
+     * Starts an animation on the stack.
+     *
+     * @param time The current time of the app in ms.
+     * @param type The type of the animation to start.
+     */
+    private void startAnimation(long time, OverviewAnimationType type) {
+        startAnimation(time, type, TabModel.INVALID_TAB_INDEX, false);
+    }
+
+    /**
+     * Starts an animation on the stack.
+     *
+     * @param time The current time of the app in ms.
+     * @param type The type of the animation to start.
+     * @param finishImmediately Whether the animation jumps straight to the end.
+     */
+    private void startAnimation(long time, OverviewAnimationType type, boolean finishImmediately) {
+        startAnimation(time, type, TabModel.INVALID_TAB_INDEX, finishImmediately);
+    }
+
+    /**
+     * Starts an animation on the stack.
+     *
+     * @param time The current time of the app in ms.
+     * @param type The type of the animation to start.
+     * @param sourceIndex The source index needed by some animation types.
+     * @param finishImmediately Whether the animation jumps straight to the end.
+     */
+    private void startAnimation(
+            long time, OverviewAnimationType type, int sourceIndex, boolean finishImmediately) {
+        startAnimation(time, type, mTabModel.index(), sourceIndex, finishImmediately);
+    }
+
+    private void startAnimation(long time, OverviewAnimationType type, int focusIndex,
+            int sourceIndex, boolean finishImmediately) {
+        if (!canUpdateAnimation(time, type, sourceIndex, finishImmediately)) {
+            // We need to finish animations started earlier before we start
+            // off a new one.
+            finishAnimation(time);
+            // Stop movement while the animation takes place.
+            stopScrollingMovement(time);
+        }
+
+        if (mAnimationFactory != null && mViewAnimationFactory != null) {
+            mOverviewAnimationType = type;
+
+            // First try to build a View animation.  Then fallback to the compositor animation if
+            // one isn't created.
+            mViewAnimations = mViewAnimationFactory.createAnimatorSetForType(
+                    type, mStackTabs, mLayout.getViewContainer(), mTabModel, focusIndex);
+
+            if (mViewAnimations != null) {
+                mViewAnimations.addListener(mViewAnimatorListener);
+            } else {
+                // Build the AnimatorSet using the TabSwitcherAnimationFactory.
+                // This will give us the appropriate AnimatorSet based on the current
+                // state of the tab switcher and the OverviewAnimationType specified.
+                mTabAnimations =
+                        mAnimationFactory.createAnimatorSetForType(type, mStackTabs, focusIndex,
+                                sourceIndex, mSpacing, mScrollOffset, mWarpSize, getDiscardRange());
+            }
+
+            if (mTabAnimations != null) mTabAnimations.start();
+            if (mViewAnimations != null) mViewAnimations.start();
+            if (mTabAnimations != null || mViewAnimations != null) {
+                mLayout.onStackAnimationStarted();
+            }
+
+            if ((mTabAnimations == null && mViewAnimations == null) || finishImmediately) {
+                finishAnimation(time);
+            }
+        }
+
+        requestUpdate();
+    }
+
+    /**
+     * Performs the necessary actions to finish the current animation.
+     *
+     * @param time The current time of the app in ms.
+     */
+    private void finishAnimation(long time) {
+        if (mTabAnimations != null) mTabAnimations.updateAndFinish();
+        if (mViewAnimations != null) mViewAnimations.end();
+        if (mTabAnimations != null || mViewAnimations != null) mLayout.onStackAnimationFinished();
+
+        switch (mOverviewAnimationType) {
+            case ENTER_STACK:
+                mLayout.uiDoneEnteringStack();
+                break;
+            case FULL_ROLL:
+                springBack(time);
+                break;
+            case TAB_FOCUSED:
+            // Purposeful fall through
+            case NEW_TAB_OPENED:
+                // Nothing to do.
+                break;
+            case DISCARD_ALL:
+                mLayout.uiDoneClosingAllTabs(mTabModel.isIncognito());
+                cleanupStackTabState();
+                break;
+            case UNDISCARD:
+            // Purposeful fall through because if UNDISCARD animation updated DISCARD animation,
+            // DISCARD animation clean up below is not called so UNDISCARD is responsible for
+            // cleaning it up.
+            case DISCARD:
+                // Remove all dying tabs from mStackTabs.
+                if (mStackTabs != null) {
+                    // Request for the model to be updated.
+                    for (int i = 0; i < mStackTabs.length; ++i) {
+                        StackTab tab = mStackTabs[i];
+                        if (tab.isDying()) {
+                            mLayout.uiDoneClosingTab(
+                                    time, tab.getId(), true, mTabModel.isIncognito());
+                        }
+                    }
+                }
+                cleanupStackTabState();
+                break;
+            default:
+                break;
+        }
+
+        if (mOverviewAnimationType != OverviewAnimationType.NONE) {
+            // sync the scrollTarget and scrollOffset;
+            setScrollTarget(mScrollOffset, true);
+            mOverviewAnimationType = OverviewAnimationType.NONE;
+        }
+        mTabAnimations = null;
+        mViewAnimations = null;
+    }
+
+    private void cleanupStackTabState() {
+        if (mStackTabs != null) {
+            // First count the number of tabs that are still alive.
+            int nNumberOfLiveTabs = 0;
+            for (int i = 0; i < mStackTabs.length; ++i) {
+                if (mStackTabs[i].isDying()) {
+                    mLayout.releaseTabLayout(mStackTabs[i].getLayoutTab());
+                } else {
+                    nNumberOfLiveTabs++;
+                }
+            }
+
+            if (nNumberOfLiveTabs == 0) {
+                // We have no more live {@link StackTab}. Just clean all tab related states.
+                cleanupTabs();
+            } else if (nNumberOfLiveTabs < mStackTabs.length) {
+                // If any tabs have died, we need to remove them from mStackTabs.
+
+                StackTab[] oldTabs = mStackTabs;
+                mStackTabs = new StackTab[nNumberOfLiveTabs];
+
+                int newIndex = 0;
+                for (int i = 0; i < oldTabs.length; ++i) {
+                    if (!oldTabs[i].isDying()) {
+                        mStackTabs[newIndex] = oldTabs[i];
+                        mStackTabs[newIndex].setNewIndex(newIndex);
+                        newIndex++;
+                    }
+                }
+                assert newIndex == nNumberOfLiveTabs;
+            }
+        }
+
+        mDiscardDirection = getDefaultDiscardDirection();
+    }
+
+    /**
+     * Ensure that there are no dying tabs by finishing the current animation.
+     *
+     * @param time The current time of the app in ms.
+     */
+    public void ensureCleaningUpDyingTabs(long time) {
+        finishAnimation(time);
+    }
+
+    /**
+     * Decide if the animation can be started without cleaning up the current animation.
+     * @param time              The current time of the app in ms.
+     * @param type              The type of the animation to start.
+     * @param sourceIndex       The source index needed by some animation types.
+     * @param finishImmediately Whether the animation jumps straight to the end.
+     * @return                  true, if we can start the animation without cleaning up the current
+     *                          animation.
+     */
+    private boolean canUpdateAnimation(
+            long time, OverviewAnimationType type, int sourceIndex, boolean finishImmediately) {
+        if (mAnimationFactory != null) {
+            if ((mOverviewAnimationType == OverviewAnimationType.DISCARD
+                        || mOverviewAnimationType == OverviewAnimationType.UNDISCARD
+                        || mOverviewAnimationType == OverviewAnimationType.DISCARD_ALL)
+                    && (type == OverviewAnimationType.DISCARD
+                               || type == OverviewAnimationType.UNDISCARD
+                               || type == OverviewAnimationType.DISCARD_ALL)) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    /**
+     * Cancel scrolling animation which is a part of discarding animation.
+     * @return true if the animation is canceled, false, if there is nothing to cancel.
+     */
+    private boolean cancelDiscardScrollingAnimation() {
+        if (mOverviewAnimationType == OverviewAnimationType.DISCARD
+                || mOverviewAnimationType == OverviewAnimationType.UNDISCARD
+                || mOverviewAnimationType == OverviewAnimationType.DISCARD_ALL) {
+            mTabAnimations.cancel(null, StackTab.Property.SCROLL_OFFSET);
+            return true;
+        }
+        return false;
+    }
+
+    /**
+     * Checks any Android view animations to see if they have finished yet.
+     * @param time      The current time of the app in ms.
+     * @param jumpToEnd Whether to finish the animation.
+     * @return          Whether the animation was finished.
+     */
+    public boolean onUpdateViewAnimation(long time, boolean jumpToEnd) {
+        boolean finished = true;
+        if (mViewAnimations != null) {
+            finished = !mViewAnimations.isRunning();
+            finishAnimationsIfDone(time, jumpToEnd);
+        }
+        return finished;
+    }
+
+    /**
+     * Steps the animation forward and updates all the animated values.
+     * @param time      The current time of the app in ms.
+     * @param jumpToEnd Whether to finish the animation.
+     * @return          Whether the animation was finished.
+     */
+    public boolean onUpdateCompositorAnimations(long time, boolean jumpToEnd) {
+        if (!jumpToEnd) updateScrollOffset(time);
+
+        boolean finished = true;
+        if (mTabAnimations != null) {
+            if (jumpToEnd) {
+                finished = mTabAnimations.finished();
+            } else {
+                finished = mTabAnimations.update(time);
+            }
+            finishAnimationsIfDone(time, jumpToEnd);
+        }
+
+        if (jumpToEnd) forceScrollStop();
+        return finished;
+    }
+
+    private void finishAnimationsIfDone(long time, boolean jumpToEnd) {
+        boolean hasViewAnimations = mViewAnimations != null;
+        boolean hasTabAnimations = mTabAnimations != null;
+        boolean hasAnimations = hasViewAnimations || hasTabAnimations;
+        boolean isViewFinished = hasViewAnimations ? !mViewAnimations.isRunning() : true;
+        boolean isTabFinished = hasTabAnimations ? mTabAnimations.finished() : true;
+
+        boolean shouldFinish = jumpToEnd && hasAnimations;
+        shouldFinish |= hasAnimations && (!hasViewAnimations || isViewFinished)
+                && (!hasTabAnimations || isTabFinished);
+        if (shouldFinish) finishAnimation(time);
+    }
+
+    /**
+     * Determines which action was specified by the user's drag.
+     *
+     * @param scrollDrag  The number of pixels moved in the scroll direction.
+     * @param discardDrag The number of pixels moved in the discard direction.
+     * @return            The current lock mode or a hint if the motion was not strong enough
+     *                    to fully lock the mode.
+     */
+    private DragLock computeDragLock(float scrollDrag, float discardDrag) {
+        scrollDrag = Math.abs(scrollDrag);
+        discardDrag = Math.abs(discardDrag);
+        DragLock hintLock = (discardDrag * DRAG_ANGLE_THRESHOLD) > scrollDrag ? DragLock.DISCARD
+                                                                              : DragLock.SCROLL;
+        // If the user paused the drag for too long, re-determine what the new action is.
+        long timeMillisecond = System.currentTimeMillis();
+        if ((timeMillisecond - mLastScrollUpdate) > DRAG_TIME_THRESHOLD) {
+            mDragLock = DragLock.NONE;
+        }
+        // Select the scroll lock if enough conviction is put into scrolling.
+        if ((mDragLock == DragLock.NONE && Math.abs(scrollDrag - discardDrag) > mMinScrollMotion)
+                || (mDragLock == DragLock.DISCARD && discardDrag > mMinScrollMotion)
+                || (mDragLock == DragLock.SCROLL && scrollDrag > mMinScrollMotion)) {
+            mLastScrollUpdate = timeMillisecond;
+            if (mDragLock == DragLock.NONE) {
+                mDragLock = hintLock;
+            }
+        }
+        // Returns a hint of the lock so we can show feedback even if the lock is not committed yet.
+        return mDragLock == DragLock.NONE ? hintLock : mDragLock;
+    }
+
+    /*
+     * User Input Routines:
+     *
+     * The input routines that process gestures and click touches.  These
+     * are the main way to interact with the view directly.  Other input
+     * paths happen when model changes impact the view.  This can happen
+     * as a result of some of these actions or from other user input (ie:
+     * from the Toolbar).  These are ignored if an animation is currently
+     * in progress.
+     */
+
+    /**
+     * Called on drag event (from scroll events in the gesture detector).
+     *
+     * @param time    The current time of the app in ms.
+     * @param x       The x coordinate of the end of the drag event.
+     * @param y       The y coordinate of the end of the drag event.
+     * @param amountX The number of pixels dragged in the x direction since the last event.
+     * @param amountY The number of pixels dragged in the y direction since the last event.
+     */
+    public void drag(long time, float x, float y, float amountX, float amountY) {
+        float scrollDrag, discardDrag;
+        if (mCurrentMode == Orientation.PORTRAIT) {
+            discardDrag = amountX;
+            scrollDrag = amountY;
+        } else {
+            discardDrag = amountY;
+            scrollDrag = LocalizationUtils.isLayoutRtl() ? -amountX : amountX;
+        }
+        DragLock hintLock = computeDragLock(scrollDrag, discardDrag);
+        if (hintLock == DragLock.DISCARD) {
+            discard(x, y, amountX, amountY);
+        } else {
+            // Only cancel the current discard attempt if the scroll lock is committed:
+            // by using mDragLock instead of hintLock.
+            if (mDragLock == DragLock.SCROLL && mDiscardingTab != null) {
+                commitDiscard(time, false);
+            }
+            scroll(x, y, LocalizationUtils.isLayoutRtl() ? -amountX : amountX, amountY, false);
+        }
+        requestUpdate();
+    }
+
+    /**
+     * Discards and updates the position based on the input event values.
+     *
+     * @param x       The x coordinate of the end of the drag event.
+     * @param y       The y coordinate of the end of the drag event.
+     * @param amountX The number of pixels dragged in the x direction since the last event.
+     * @param amountY The number of pixels dragged in the y direction since the last event.
+     */
+    private void discard(float x, float y, float amountX, float amountY) {
+        if (mStackTabs == null
+                || (mOverviewAnimationType != OverviewAnimationType.NONE
+                           && mOverviewAnimationType != OverviewAnimationType.DISCARD
+                           && mOverviewAnimationType != OverviewAnimationType.DISCARD_ALL
+                           && mOverviewAnimationType != OverviewAnimationType.UNDISCARD)) {
+            return;
+        }
+
+        if (mDiscardingTab == null) {
+            if (!mInSwipe) {
+                mDiscardingTab = getTabAtPositon(x, y);
+            } else {
+                if (mTabModel.index() < 0) return;
+                mDiscardingTab = mStackTabs[mTabModel.index()];
+            }
+
+            if (mDiscardingTab != null) {
+                cancelDiscardScrollingAnimation();
+
+                // Make sure we are well within the tab in the discard direction.
+                RectF target = mDiscardingTab.getLayoutTab().getClickTargetBounds();
+                float distanceToEdge;
+                float edgeToEdge;
+                if (mCurrentMode == Orientation.PORTRAIT) {
+                    mDiscardDirection = 1.0f;
+                    distanceToEdge = Math.max(target.left - x, x - target.right);
+                    edgeToEdge = target.width();
+                } else {
+                    mDiscardDirection = 2.0f - 4.0f * (x / mLayout.getWidth());
+                    mDiscardDirection = MathUtils.clamp(mDiscardDirection, -1.0f, 1.0f);
+                    distanceToEdge = Math.max(target.top - y, y - target.bottom);
+                    edgeToEdge = target.height();
+                }
+
+                float scaledDiscardX = x - mDiscardingTab.getLayoutTab().getX();
+                float scaledDiscardY = y - mDiscardingTab.getLayoutTab().getY();
+                mDiscardingTab.setDiscardOriginX(scaledDiscardX / mDiscardingTab.getScale());
+                mDiscardingTab.setDiscardOriginY(scaledDiscardY / mDiscardingTab.getScale());
+                mDiscardingTab.setDiscardFromClick(false);
+
+                if (Math.abs(distanceToEdge) < DISCARD_SAFE_SELECTION_PCTG * edgeToEdge) {
+                    mDiscardingTab = null;
+                }
+            }
+        }
+        if (mDiscardingTab != null) {
+            float deltaAmount = mCurrentMode == Orientation.PORTRAIT ? amountX : amountY;
+            mDiscardingTab.addToDiscardAmount(deltaAmount);
+        }
+    }
+
+    /**
+     * Called on touch/tilt scroll event.
+     *
+     * @param x       The x coordinate of the end of the scroll event.
+     * @param y       The y coordinate of the end of the scroll event.
+     * @param amountX The number of pixels scrolled in the x direction.
+     * @param amountY The number of pixels scrolled in the y direction.
+     * @param isTilt  True if the call comes from a tilt event.
+     */
+    private void scroll(float x, float y, float amountX, float amountY, boolean isTilt) {
+        if ((!mScroller.isFinished() && isTilt) || mStackTabs == null
+                || (mOverviewAnimationType != OverviewAnimationType.NONE
+                           && mOverviewAnimationType != OverviewAnimationType.DISCARD
+                           && mOverviewAnimationType != OverviewAnimationType.UNDISCARD
+                           && mOverviewAnimationType != OverviewAnimationType.DISCARD_ALL
+                           && mOverviewAnimationType != OverviewAnimationType.ENTER_STACK)) {
+            return;
+        }
+
+        float amountScreen = mCurrentMode == Orientation.PORTRAIT ? amountY : amountX;
+        float amountScroll = amountScreen;
+        float amountEvenOut = amountScreen;
+
+        // Computes the right amount for the scrolling so the finger matches the tab under it.
+        float tabScrollSpaceFinal = 0;
+        if (mScrollingTab == null || isTilt) {
+            mScrollingTab = getTabAtPositon(x, y);
+        }
+
+        if (mScrollingTab == null && mInSwipe && mStackTabs != null) {
+            int index = mTabModel.index();
+            if (index >= 0 && index <= mStackTabs.length) mScrollingTab = mStackTabs[index];
+        }
+
+        if (mScrollingTab == null) {
+            if (!isTilt) {
+                amountScroll = 0;
+                amountEvenOut = 0;
+            }
+        } else if (mScrollingTab.getIndex() == 0) {
+            amountEvenOut = 0;
+        } else {
+            // Find the scroll that make the selected tab move the right
+            // amount on the screen.
+            float tabScrollSpace = mScrollingTab.getScrollOffset() + mScrollOffset;
+            float tabScreen = scrollToScreen(tabScrollSpace);
+            tabScrollSpaceFinal = screenToScroll(tabScreen + amountScreen);
+            amountScroll = tabScrollSpaceFinal - tabScrollSpace;
+            // Matching the finger is too strong of a constraints on the edges. So we make
+            // sure the end value is not too far from the linear case.
+            amountScroll = Math.signum(amountScreen)
+                    * MathUtils.clamp(Math.abs(amountScroll), Math.abs(amountScreen) * 0.5f,
+                              Math.abs(amountScreen) * 2.0f);
+        }
+
+        // Evens out the tabs and correct the scroll amount if needed.
+        if (evenOutTabs(amountEvenOut, false) && mScrollingTab.getIndex() > 0) {
+            // Adjust the amount after the even phase
+            float tabScrollSpace = mScrollingTab.getScrollOffset() + mScrollOffset;
+            amountScroll = tabScrollSpaceFinal - tabScrollSpace;
+        }
+
+        // Actually do the scrolling.
+        setScrollTarget(mScrollTarget + amountScroll, false);
+    }
+
+    /**
+     * Evens out auto-magically the cards as the stack get scrolled.
+     *
+     * @param amount                The amount of scroll performed in pixel. The sign indicates the
+     *                              direction.
+     * @param allowReverseDirection Whether or not to allow corrections in the reverse direction of
+     *                              the amount scrolled.
+     * @return                      True if any tab had been 'visibly' moved.
+     */
+    private boolean evenOutTabs(float amount, boolean allowReverseDirection) {
+        if (mStackTabs == null || mOverviewAnimationType != OverviewAnimationType.NONE
+                || mEvenOutProgress >= 1.0f || amount == 0) {
+            return false;
+        }
+        boolean changed = false;
+        boolean reverseScrolling = false;
+
+        // The evening out process last until mEvenOutRate reaches 1.0. Tabs blend linearly
+        // between the current position to a nice evenly scaled pattern. Because we do not store
+        // the starting position for each tab we need more complicated math to do the blend.
+        // The absoluteProgress is how much we need progress this step on the [0, 1] scale.
+        float absoluteProgress = Math.min(Math.abs(amount) * mEvenOutRate, 1.0f - mEvenOutProgress);
+        // The relativeProgress is how much we need to blend the target to the current to get there.
+        float relativeProgress = absoluteProgress / (1.0f - mEvenOutProgress);
+
+        float screenMax = getScrollDimensionSize();
+        for (int i = 0; i < mStackTabs.length; ++i) {
+            float source = mStackTabs[i].getScrollOffset();
+            float target = screenToScroll(i * mSpacing);
+            float sourceScreen = Math.min(screenMax, scrollToScreen(source + mScrollTarget));
+            float targetScreen = Math.min(screenMax, scrollToScreen(target + mScrollTarget));
+            // If the target and the current position matches on the screen then we snap to the
+            // target.
+            if (sourceScreen == targetScreen) {
+                mStackTabs[i].setScrollOffset(target);
+                continue;
+            }
+            float step = source + (target - source) * relativeProgress;
+            float stepScreen = Math.min(screenMax, scrollToScreen(step + mScrollTarget));
+            // If the step can be performed without noticing then we do it.
+            if (sourceScreen == stepScreen) {
+                mStackTabs[i].setScrollOffset(step);
+                continue;
+            }
+            // If the scrolling goes in the same direction as the step then the motion is applied.
+            if ((targetScreen - sourceScreen) * amount > 0 || allowReverseDirection) {
+                mStackTabs[i].setScrollOffset(step);
+                changed = true;
+            } else {
+                reverseScrolling = true;
+            }
+        }
+        // Only account for progress if the scrolling was in the right direction. It assumes here
+        // That if any of the tabs was going in the wrong direction then the progress is not
+        // recorded at all. This is very conservative to avoid poping in the scrolling. It works
+        // for now but might need to be revisited if we see artifacts.
+        if (!reverseScrolling) {
+            mEvenOutProgress += absoluteProgress;
+        }
+        return changed;
+    }
+
+    /**
+     * Called on touch fling event. Scroll the stack or help to discard a tab.
+     *
+     * @param time      The current time of the app in ms.
+     * @param x         The y coordinate of the start of the fling event.
+     * @param y         The y coordinate of the start of the fling event.
+     * @param velocityX The amount of velocity in the x direction.
+     * @param velocityY The amount of velocity in the y direction.
+     */
+    public void fling(long time, float x, float y, float velocityX, float velocityY) {
+        if (mDragLock != DragLock.SCROLL && mDiscardingTab != null) {
+            float velocity = mCurrentMode == Orientation.PORTRAIT ? velocityX : velocityY;
+            float maxDelta = getDiscardRange() * DISCARD_FLING_MAX_CONTRIBUTION;
+            float deltaAmount = MathUtils.clamp(velocity * DISCARD_FLING_DT, -maxDelta, maxDelta);
+            mDiscardingTab.addToDiscardAmount(deltaAmount);
+        } else if (mOverviewAnimationType == OverviewAnimationType.NONE && mScroller.isFinished()
+                && mOverScrollOffset == 0 && getTabIndexAtPositon(x, y) >= 0) {
+            float velocity = mCurrentMode == Orientation.PORTRAIT
+                    ? velocityY
+                    : (LocalizationUtils.isLayoutRtl() ? -velocityX : velocityX);
+            // Fling only overscrolls when the stack is fully unfolded.
+            mScroller.fling(0, (int) mScrollTarget, 0, (int) velocity, 0, 0,
+                    (int) getMinScroll(false), (int) getMaxScroll(false), 0,
+                    (int) ((velocity > 0 ? mMaxOverScroll : mMaxUnderScroll)
+                                    * MAX_OVER_FLING_SCALE),
+                    time);
+
+            // Set the target to the final scroll position to make sure
+            // the offset finally gets there regardless of what happens.
+            // We override this when the user interrupts the fling though.
+            setScrollTarget(mScroller.getFinalY(), false);
+        }
+    }
+
+    /**
+     * Get called on down touch event.
+     *
+     * @param time The current time of the app in ms.
+     */
+    public void onDown(long time) {
+        mDragLock = DragLock.NONE;
+        if (mOverviewAnimationType == OverviewAnimationType.NONE) {
+            stopScrollingMovement(time);
+        }
+        // Resets the scrolling state.
+        mScrollingTab = null;
+        commitDiscard(time, false);
+    }
+
+    /**
+     * Get called on long press touch event.
+     *
+     * @param time The current time of the app in ms.
+     * @param x The x coordinate in pixel inside the stack view.
+     * @param y The y coordinate in pixel inside the stack view.
+     */
+    public void onLongPress(long time, float x, float y) {
+        if (mOverviewAnimationType == OverviewAnimationType.NONE) {
+            mLongPressSelected = getTabIndexAtPositon(x, y);
+            if (mLongPressSelected >= 0) {
+                startAnimation(time, OverviewAnimationType.VIEW_MORE, mLongPressSelected, false);
+                mEvenOutProgress = 0.0f;
+            }
+        }
+    }
+
+    /**
+     * Called when at least 2 touch events are detected.
+     *
+     * @param time       The current time of the app in ms.
+     * @param x0         The x coordinate of the first touch event.
+     * @param y0         The y coordinate of the first touch event.
+     * @param x1         The x coordinate of the second touch event.
+     * @param y1         The y coordinate of the second touch event.
+     * @param firstEvent The pinch is the first of a sequence of pinch events.
+     */
+    public void onPinch(long time, float x0, float y0, float x1, float y1, boolean firstEvent) {
+        if ((mOverviewAnimationType != OverviewAnimationType.START_PINCH
+                && mOverviewAnimationType != OverviewAnimationType.NONE) || mStackTabs == null) {
+            return;
+        }
+        if (mPinch0TabIndex < 0) startAnimation(time, OverviewAnimationType.START_PINCH);
+
+        // Reordering the fingers so pinch0 is always the closest to the top of the stack.
+        // This allows simpler math down the line where we assume that
+        // pinch0TabIndex <= pinch0TabIndex
+        // It also means that crossing the finger will separate the tabs again.
+        boolean inverse = (mCurrentMode == Orientation.PORTRAIT)
+                ? y0 > y1
+                : LocalizationUtils.isLayoutRtl() ? (x0 <= x1) : (x0 > x1);
+        float pinch0X = inverse ? x1 : x0;
+        float pinch0Y = inverse ? y1 : y0;
+        float pinch1X = inverse ? x0 : x1;
+        float pinch1Y = inverse ? y0 : y1;
+        float pinch0Offset = (mCurrentMode == Orientation.PORTRAIT)
+                ? pinch0Y
+                : LocalizationUtils.isLayoutRtl() ? -pinch0X : pinch0X;
+        float pinch1Offset = (mCurrentMode == Orientation.PORTRAIT)
+                ? pinch1Y
+                : LocalizationUtils.isLayoutRtl() ? -pinch1X : pinch1X;
+
+        if (firstEvent) {
+            // Resets pinch and scrolling state.
+            mPinch0TabIndex = -1;
+            mPinch1TabIndex = -1;
+            mScrollingTab = null;
+            commitDiscard(time, false);
+        }
+        int pinch0TabIndex = mPinch0TabIndex;
+        int pinch1TabIndex = mPinch1TabIndex;
+        if (mPinch0TabIndex < 0) {
+            pinch0TabIndex = getTabIndexAtPositon(pinch0X, pinch0Y);
+            pinch1TabIndex = getTabIndexAtPositon(pinch1X, pinch1Y);
+            // If any of them is invalid we invalidate both.
+            if (pinch0TabIndex < 0 || pinch1TabIndex < 0) {
+                pinch0TabIndex = -1;
+                pinch1TabIndex = -1;
+            }
+        }
+
+        if (pinch0TabIndex >= 0 && mPinch0TabIndex == pinch0TabIndex
+                && mPinch1TabIndex == pinch1TabIndex) {
+            final float minScrollTarget = getMinScroll(false);
+            final float maxScrollTarget = getMaxScroll(false);
+            final float oldScrollTarget =
+                    MathUtils.clamp(mScrollTarget, minScrollTarget, maxScrollTarget);
+            // pinch0TabIndex > pinch1TabIndex is unexpected but we do not want to exit
+            // ungracefully so process it as if the tabs were the same.
+            if (pinch0TabIndex >= pinch1TabIndex) {
+                // If one tab is pinched then we only scroll.
+                float screenDelta0 = pinch0Offset - mLastPinch0Offset;
+                if (pinch0TabIndex == 0) {
+                    // Linear scroll on the top tab for the overscroll to kick-in linearly.
+                    setScrollTarget(oldScrollTarget + screenDelta0, false);
+                } else {
+                    float tab0ScrollSpace =
+                            mStackTabs[pinch0TabIndex].getScrollOffset() + oldScrollTarget;
+                    float tab0Screen = scrollToScreen(tab0ScrollSpace);
+                    float tab0ScrollFinal = screenToScroll(tab0Screen + screenDelta0);
+                    setScrollTarget(
+                            tab0ScrollFinal - mStackTabs[pinch0TabIndex].getScrollOffset(), false);
+                }
+                // This is the common case of the pinch, 2 fingers on 2 different tabs.
+            } else {
+                // Find the screen space position before and after the scroll so the tab 0 matches
+                // the finger 0 motion.
+                float screenDelta0 = pinch0Offset - mLastPinch0Offset;
+                float tab0ScreenBefore = approxScreen(mStackTabs[pinch0TabIndex], oldScrollTarget);
+                float tab0ScreenAfter = tab0ScreenBefore + screenDelta0;
+
+                // Find the screen space position before and after the scroll so the tab 1 matches
+                // the finger 1 motion.
+                float screenDelta1 = pinch1Offset - mLastPinch1Offset;
+                float tab1ScreenBefore = approxScreen(mStackTabs[pinch1TabIndex], oldScrollTarget);
+                float tab1ScreenAfter = tab1ScreenBefore + screenDelta1;
+
+                // Heuristic: the scroll is defined by half the change of the first pinched tab.
+                // The rational is that it looks nice this way :)... Scrolling creates a sliding
+                // effect. When a finger does not move then it is expected that none of the tabs
+                // past that steady finger should move. This does the job.
+                float globalScrollBefore = screenToScroll(tab0ScreenBefore);
+                float globalScrollAfter = screenToScroll((tab0ScreenAfter + tab0ScreenBefore) / 2);
+                setScrollTarget(oldScrollTarget + globalScrollAfter - globalScrollBefore, true);
+
+                // Evens out the tabs in between
+                float minScreen = tab0ScreenAfter;
+                float maxScreen = tab0ScreenAfter;
+                for (int i = pinch0TabIndex; i <= pinch1TabIndex; i++) {
+                    float screenBefore = approxScreen(mStackTabs[i], oldScrollTarget);
+                    float t = (screenBefore - tab0ScreenBefore)
+                            / (tab1ScreenBefore - tab0ScreenBefore);
+                    float screenAfter = (1 - t) * tab0ScreenAfter + t * tab1ScreenAfter;
+                    screenAfter = Math.max(minScreen, screenAfter);
+                    screenAfter = Math.min(maxScreen, screenAfter);
+                    minScreen = screenAfter + StackTab.sStackedTabVisibleSize;
+                    maxScreen = screenAfter + mStackTabs[i].getSizeInScrollDirection(mCurrentMode);
+                    float newScrollOffset = screenToScroll(screenAfter) - mScrollTarget;
+                    mStackTabs[i].setScrollOffset(newScrollOffset);
+                }
+
+                // Push a bit the tabs bellow pinch1.
+                float delta1 = tab1ScreenAfter - tab1ScreenBefore;
+                for (int i = pinch1TabIndex + 1; i < mStackTabs.length; i++) {
+                    delta1 /= 2;
+                    float screenAfter = approxScreen(mStackTabs[i], oldScrollTarget) + delta1;
+                    screenAfter = Math.max(minScreen, screenAfter);
+                    screenAfter = Math.min(maxScreen, screenAfter);
+                    minScreen = screenAfter + StackTab.sStackedTabVisibleSize;
+                    maxScreen = screenAfter + mStackTabs[i].getSizeInScrollDirection(mCurrentMode);
+                    mStackTabs[i].setScrollOffset(screenToScroll(screenAfter) - mScrollTarget);
+                }
+
+                // Pull a bit the tabs above pinch0.
+                minScreen = tab0ScreenAfter;
+                maxScreen = tab0ScreenAfter;
+                float posScreen = tab0ScreenAfter;
+                float delta0 = tab0ScreenAfter - tab0ScreenBefore;
+                for (int i = pinch0TabIndex - 1; i > 0; i--) {
+                    delta0 /= 2;
+                    minScreen = posScreen - mStackTabs[i].getSizeInScrollDirection(mCurrentMode);
+                    maxScreen = posScreen - StackTab.sStackedTabVisibleSize;
+                    float screenAfter = approxScreen(mStackTabs[i], oldScrollTarget) + delta0;
+                    screenAfter = Math.max(minScreen, screenAfter);
+                    screenAfter = Math.min(maxScreen, screenAfter);
+                    mStackTabs[i].setScrollOffset(screenToScroll(screenAfter) - mScrollTarget);
+                }
+            }
+        }
+        mPinch0TabIndex = pinch0TabIndex;
+        mPinch1TabIndex = pinch1TabIndex;
+        mLastPinch0Offset = pinch0Offset;
+        mLastPinch1Offset = pinch1Offset;
+        mEvenOutProgress = 0.0f;
+        requestUpdate();
+    }
+
+    /**
+     * Commits or release the that currently being considered for discard. This function
+     * also triggers the associated animations.
+     *
+     * @param time         The current time of the app in ms.
+     * @param allowDiscard Whether to allow to discard the tab currently being considered
+     *                     for discard.
+     */
+    private void commitDiscard(long time, boolean allowDiscard) {
+        if (mDiscardingTab == null) return;
+
+        assert mStackTabs != null;
+        StackTab discarded = mDiscardingTab;
+        if (Math.abs(discarded.getDiscardAmount()) / getDiscardRange() > DISCARD_COMMIT_THRESHOLD
+                && allowDiscard) {
+            mLayout.uiRequestingCloseTab(time, discarded.getId());
+            RecordUserAction.record("MobileStackViewSwipeCloseTab");
+            RecordUserAction.record("MobileTabClosed");
+        } else {
+            startAnimation(time, OverviewAnimationType.UNDISCARD);
+        }
+        mDiscardingTab = null;
+        requestUpdate();
+    }
+
+    /**
+     * Called on touch up or cancel event.
+     */
+    public void onUpOrCancel(long time) {
+        // Make sure the bottom tab always goes back to the top of the screen.
+        if (mPinch0TabIndex >= 0) {
+            startAnimation(time, OverviewAnimationType.REACH_TOP);
+            requestUpdate();
+        }
+        // Commit or uncommit discard tab
+        commitDiscard(time, true);
+
+        resetInputActionIndices();
+
+        springBack(time);
+    }
+
+    /**
+     * Bounces back if we happen to overscroll the stack.
+     */
+    private void springBack(long time) {
+        if (mScroller.isFinished()) {
+            int minScroll = (int) getMinScroll(false);
+            int maxScroll = (int) getMaxScroll(false);
+            if (mScrollTarget < minScroll || mScrollTarget > maxScroll) {
+                mScroller.springBack(0, (int) mScrollTarget, 0, 0, minScroll, maxScroll, time);
+                setScrollTarget(MathUtils.clamp(mScrollTarget, minScroll, maxScroll), false);
+                requestUpdate();
+            }
+        }
+    }
+
+    /**
+     * Called on touch click event.
+     *
+     * @param time The current time of the app in ms.
+     * @param x    The x coordinate in pixel inside the stack view.
+     * @param y    The y coordinate in pixel inside the stack view.
+     */
+    public void click(long time, float x, float y) {
+        if (mOverviewAnimationType != OverviewAnimationType.NONE
+                && mOverviewAnimationType != OverviewAnimationType.DISCARD
+                && mOverviewAnimationType != OverviewAnimationType.UNDISCARD
+                && mOverviewAnimationType != OverviewAnimationType.DISCARD_ALL) {
+            return;
+        }
+        int clicked = getTabIndexAtPositon(x, y, LayoutTab.getTouchSlop());
+        if (clicked >= 0) {
+            // Check if the click was within the boundaries of the close button defined by its
+            // visible coordinates.
+            boolean isRtl =
+                    !((mCurrentMode == Orientation.PORTRAIT) ^ LocalizationUtils.isLayoutRtl());
+            if (mStackTabs[clicked].getLayoutTab().checkCloseHitTest(x, y, isRtl)) {
+                // Tell the model to close the tab because the close button was pressed.  The model
+                // will then trigger a notification which will start the actual close process here
+                // if necessary.
+                StackTab tab = mStackTabs[clicked];
+                final float halfCloseBtnWidth = LayoutTab.CLOSE_BUTTON_WIDTH_DP / 2.f;
+                final float halfCloseBtnHeight = mBorderTopPadding / 2.f;
+                final float contentWidth = tab.getLayoutTab().getOriginalContentWidth();
+
+                tab.setDiscardOriginY(halfCloseBtnHeight);
+                tab.setDiscardOriginX(isRtl ? halfCloseBtnWidth : contentWidth - halfCloseBtnWidth);
+                tab.setDiscardFromClick(true);
+                mLayout.uiRequestingCloseTab(time, tab.getId());
+                RecordUserAction.record("MobileStackViewCloseTab");
+                RecordUserAction.record("MobileTabClosed");
+            } else {
+                // Let the model know that a new {@link LayoutTab} was selected. The model will
+                // notify us if we need to do anything visual. setIndex() will possibly switch the
+                // models and broadcast the event.
+                mLayout.uiSelectingTab(time, mStackTabs[clicked].getId());
+            }
+        }
+    }
+
+    /*
+     * Initialization and Utility Methods
+     */
+
+    /**
+     * @param context The current Android's context.
+     */
+    public void contextChanged(Context context) {
+        Resources res = context.getResources();
+        final float pxToDp = 1.0f / res.getDisplayMetrics().density;
+
+        mMinScrollMotion = DRAG_MOTION_THRESHOLD_DP;
+        final float maxOverScrollPx = res.getDimensionPixelOffset(R.dimen.over_scroll);
+        final float maxUnderScrollPx = Math.round(maxOverScrollPx * MAX_UNDER_SCROLL_SCALE);
+        mMaxOverScroll = maxOverScrollPx * pxToDp;
+        mMaxUnderScroll = maxUnderScrollPx * pxToDp;
+        mMaxOverScrollAngle = res.getInteger(R.integer.over_scroll_angle);
+        mMaxOverScrollSlide = res.getDimensionPixelOffset(R.dimen.over_scroll_slide) * pxToDp;
+        mEvenOutRate = 1.0f / (res.getDimension(R.dimen.even_out_scrolling) * pxToDp);
+        mMinSpacing = res.getDimensionPixelOffset(R.dimen.min_spacing) * pxToDp;
+        mBorderTransparentTop =
+                res.getDimension(R.dimen.tabswitcher_border_frame_transparent_top) * pxToDp;
+        mBorderTransparentSide =
+                res.getDimension(R.dimen.tabswitcher_border_frame_transparent_side) * pxToDp;
+        mBorderTopPadding = res.getDimension(R.dimen.tabswitcher_border_frame_padding_top) * pxToDp;
+        mBorderLeftPadding =
+                res.getDimension(R.dimen.tabswitcher_border_frame_padding_left) * pxToDp;
+
+        // Just in case the density has changed, rebuild the OverScroller.
+        mScroller = new StackScroller(context);
+    }
+
+    /**
+     * @param width       The new width of the layout.
+     * @param height      The new height of the layout.
+     * @param orientation The new orientation of the layout.
+     */
+    public void notifySizeChanged(float width, float height, int orientation) {
+        updateCurrentMode(orientation);
+    }
+
+    private float getScrollDimensionSize() {
+        return mCurrentMode == Orientation.PORTRAIT ? mLayout.getHeightMinusTopControls()
+                                                    : mLayout.getWidth();
+    }
+
+    /**
+     * Gets the tab instance at the requested position.
+     *
+     * @param x The x coordinate where to perform the hit test.
+     * @param y The y coordinate where to perform the hit test.
+     * @return  The instance of the tab selected. null if none.
+     */
+    private StackTab getTabAtPositon(float x, float y) {
+        int tabIndexAtPosition = getTabIndexAtPositon(x, y, 0);
+        return tabIndexAtPosition < 0 ? null : mStackTabs[tabIndexAtPosition];
+    }
+
+    /**
+     * Gets the tab index at the requested position.
+     *
+     * @param x The x coordinate where to perform the hit test.
+     * @param y The y coordinate where to perform the hit test.
+     * @return  The index of the tab selected. -1 if none.
+     */
+    private int getTabIndexAtPositon(float x, float y) {
+        return getTabIndexAtPositon(x, y, 0);
+    }
+
+    /**
+     * Gets the tab index at the requested position.
+     *
+     * @param x    The x coordinate where to perform the hit test.
+     * @param y    The y coordinate where to perform the hit test.
+     * @param slop The acceptable distance to a tab for it to be considered.
+     * @return     The index of the tab selected. -1 if none.
+     */
+    private int getTabIndexAtPositon(float x, float y, float slop) {
+        int closestIndex = -1;
+        float closestDistance = mLayout.getHeight() + mLayout.getWidth();
+        if (mStackTabs != null) {
+            for (int i = mStackTabs.length - 1; i >= 0; --i) {
+                // This is a fail safe.  We should never have a situation where a dying
+                // {@link LayoutTab} can get accessed (the animation check should catch it).
+                if (!mStackTabs[i].isDying() && mStackTabs[i].getLayoutTab().isVisible()) {
+                    float d = mStackTabs[i].getLayoutTab().computeDistanceTo(x, y);
+                    // Strict '<' is very important here because we might have several tab at the
+                    // same place and we want the one above.
+                    if (d < closestDistance) {
+                        closestIndex = i;
+                        closestDistance = d;
+                        if (d == 0) break;
+                    }
+                }
+            }
+        }
+        return closestDistance <= slop ? closestIndex : -1;
+    }
+
+    /**
+     * ComputeTabPosition pass 1:
+     * Combine the overall stack scale with the animated tab scale.
+     *
+     * @param stackRect The frame of the stack.
+     */
+    private void computeTabScaleAlphaDepthHelper(RectF stackRect) {
+        final float stackScale = getStackScale(stackRect);
+        final float discardRange = getDiscardRange();
+
+        for (int i = 0; i < mStackTabs.length; ++i) {
+            assert mStackTabs[i] != null;
+            StackTab stackTab = mStackTabs[i];
+            LayoutTab layoutTab = stackTab.getLayoutTab();
+            final float discard = stackTab.getDiscardAmount();
+
+            // Scale
+            float discardScale =
+                    computeDiscardScale(discard, discardRange, stackTab.getDiscardFromClick());
+            layoutTab.setScale(stackTab.getScale() * discardScale * stackScale);
+            layoutTab.setBorderScale(discardScale);
+
+            // Alpha
+            float discardAlpha = computeDiscardAlpha(discard, discardRange);
+            layoutTab.setAlpha(stackTab.getAlpha() * discardAlpha);
+        }
+    }
+
+    /**
+     * ComputeTabPosition pass 2:
+     * Adjust the scroll offsets of each tab so no there is no void in between tabs.
+     */
+    private void computeTabScrollOffsetHelper() {
+        float maxScrollOffset = Float.MAX_VALUE;
+        for (int i = 0; i < mStackTabs.length; ++i) {
+            if (mStackTabs[i].isDying()) continue;
+
+            float tabScrollOffset = Math.min(maxScrollOffset, mStackTabs[i].getScrollOffset());
+            mStackTabs[i].setScrollOffset(tabScrollOffset);
+
+            float maxScreenScrollOffset = scrollToScreen(mScrollOffset + tabScrollOffset);
+            maxScrollOffset = -mScrollOffset
+                    + screenToScroll(maxScreenScrollOffset
+                                      + mStackTabs[i].getSizeInScrollDirection(mCurrentMode));
+        }
+    }
+
+    /**
+     * ComputeTabPosition pass 3:
+     * Compute the position of the tabs. Adjust for top and bottom stacking.
+     *
+     * @param stackRect The frame of the stack.
+     */
+    private void computeTabOffsetHelper(RectF stackRect) {
+        final boolean portrait = mCurrentMode == Orientation.PORTRAIT;
+
+        // Precompute the position using scroll offset and top stacking.
+        final float parentWidth = stackRect.width();
+        final float parentHeight = stackRect.height();
+        final float overscrollPercent = computeOverscrollPercent();
+        final float scrollOffset =
+                MathUtils.clamp(mScrollOffset, getMinScroll(false), getMaxScroll(false));
+        final float stackScale = getStackScale(stackRect);
+
+        int stackedCount = 0;
+        float minStackedPosition = 0.0f;
+        for (int i = 0; i < mStackTabs.length; ++i) {
+            assert mStackTabs[i] != null;
+            StackTab stackTab = mStackTabs[i];
+            LayoutTab layoutTab = stackTab.getLayoutTab();
+
+            // Position
+            final float stackScrollOffset =
+                    stackTab.isDying() ? mScrollOffsetForDyingTabs : scrollOffset;
+            float screenScrollOffset = approxScreen(stackTab, stackScrollOffset);
+
+            // Resolve top stacking
+            screenScrollOffset = Math.max(minStackedPosition, screenScrollOffset);
+            if (stackedCount < MAX_NUMBER_OF_STACKED_TABS_TOP) {
+                // This make sure all the tab get stacked up as one when all the tabs do a
+                // full roll animation.
+                final float tiltXcos = (float) Math.cos(Math.toRadians(layoutTab.getTiltX()));
+                final float tiltYcos = (float) Math.cos(Math.toRadians(layoutTab.getTiltY()));
+                float collapse = Math.min(Math.abs(tiltXcos), Math.abs(tiltYcos));
+                collapse *= layoutTab.getAlpha();
+                minStackedPosition += StackTab.sStackedTabVisibleSize * collapse;
+            }
+            stackedCount += stackTab.isDying() ? 0 : 1;
+            if (overscrollPercent < 0) {
+                // Oversroll at the top of the screen. For the first
+                // OVERSCROLL_TOP_SLIDE_PCTG of the overscroll, slide the tabs
+                // together so they completely overlap.  After that, stop scrolling the tabs.
+                screenScrollOffset +=
+                        (overscrollPercent / OVERSCROLL_TOP_SLIDE_PCTG) * screenScrollOffset;
+                screenScrollOffset = Math.max(0, screenScrollOffset);
+            }
+
+            // Note: All the Offsets except for centering shouldn't depend on the tab's scaling
+            //       because it interferes the scaling center.
+
+            // Centers the tab in its parent.
+            float xIn = (parentWidth - layoutTab.getScaledContentWidth()) / 2.0f;
+            float yIn = (parentHeight - layoutTab.getScaledContentHeight()) / 2.0f;
+
+            // We want slight offset from the center so that multiple tab browsing
+            // have more space to its expanding direction. e.g., On portrait mode,
+            // there will be more space on the bottom than top.
+            final float horizontalPadding =
+                    (parentWidth
+                            - layoutTab.getOriginalContentWidth() * StackAnimation.SCALE_AMOUNT
+                                    * stackScale) / 2.0f;
+            final float verticalPadding =
+                    (parentHeight
+                            - layoutTab.getOriginalContentHeight() * StackAnimation.SCALE_AMOUNT
+                                    * stackScale) / 2.0f;
+
+            if (portrait) {
+                yIn += STACK_PORTRAIT_Y_OFFSET_PROPORTION * verticalPadding;
+                yIn += screenScrollOffset;
+            } else {
+                if (LocalizationUtils.isLayoutRtl()) {
+                    xIn -= STACK_LANDSCAPE_START_OFFSET_PROPORTION * horizontalPadding;
+                    xIn -= screenScrollOffset;
+                } else {
+                    xIn += STACK_LANDSCAPE_START_OFFSET_PROPORTION * horizontalPadding;
+                    xIn += screenScrollOffset;
+                }
+                yIn += STACK_LANDSCAPE_Y_OFFSET_PROPORTION * verticalPadding;
+            }
+
+            layoutTab.setX(xIn);
+            layoutTab.setY(yIn);
+        }
+
+        // Resolve bottom stacking
+        stackedCount = 0;
+        float maxStackedPosition =
+                portrait ? mLayout.getHeightMinusTopControls() : mLayout.getWidth();
+        for (int i = mStackTabs.length - 1; i >= 0; i--) {
+            assert mStackTabs[i] != null;
+            StackTab stackTab = mStackTabs[i];
+            LayoutTab layoutTab = stackTab.getLayoutTab();
+            if (stackTab.isDying()) continue;
+
+            float pos;
+            if (portrait) {
+                pos = layoutTab.getY();
+                layoutTab.setY(Math.min(pos, maxStackedPosition));
+            } else if (LocalizationUtils.isLayoutRtl()) {
+                // On RTL landscape, pos is a distance between tab's right and mLayout's right.
+                float posOffset = mLayout.getWidth()
+                        - layoutTab.getOriginalContentWidth() * StackAnimation.SCALE_AMOUNT
+                                * stackScale;
+                pos = -layoutTab.getX() + posOffset;
+                layoutTab.setX(-Math.min(pos, maxStackedPosition) + posOffset);
+            } else {
+                pos = layoutTab.getX();
+                layoutTab.setX(Math.min(pos, maxStackedPosition));
+            }
+            if (pos >= maxStackedPosition && stackedCount < MAX_NUMBER_OF_STACKED_TABS_BOTTOM) {
+                maxStackedPosition -= StackTab.sStackedTabVisibleSize;
+                stackedCount++;
+            }
+        }
+
+        // final position blend
+        final float discardRange = getDiscardRange();
+        for (int i = 0; i < mStackTabs.length; ++i) {
+            assert mStackTabs[i] != null;
+            StackTab stackTab = mStackTabs[i];
+            LayoutTab layoutTab = stackTab.getLayoutTab();
+
+            final float xIn = layoutTab.getX() + stackTab.getXInStackOffset();
+            final float yIn = layoutTab.getY() + stackTab.getYInStackOffset();
+            final float xOut = stackTab.getXOutOfStack();
+            final float yOut = stackTab.getYOutOfStack();
+            float x = MathUtils.interpolate(xOut, xIn, stackTab.getXInStackInfluence());
+            float y = MathUtils.interpolate(yOut, yIn, stackTab.getYInStackInfluence());
+
+            // Discard offsets
+            if (stackTab.getDiscardAmount() != 0) {
+                float discard = stackTab.getDiscardAmount();
+                boolean fromClick = stackTab.getDiscardFromClick();
+                float scale = computeDiscardScale(discard, discardRange, fromClick);
+                float deltaX = stackTab.getDiscardOriginX()
+                        - stackTab.getLayoutTab().getOriginalContentWidth() / 2.f;
+                float deltaY = stackTab.getDiscardOriginY()
+                        - stackTab.getLayoutTab().getOriginalContentHeight() / 2.f;
+                float discardOffset = fromClick ? 0.f : discard;
+                if (portrait) {
+                    x += discardOffset + deltaX * (1.f - scale);
+                    y += deltaY * (1.f - scale);
+                } else {
+                    x += deltaX * (1.f - scale);
+                    y += discardOffset + deltaY * (1.f - scale);
+                }
+            }
+
+            // Finally apply the stack translation
+            layoutTab.setX(stackRect.left + x);
+            layoutTab.setY(stackRect.top + y);
+        }
+    }
+
+    /**
+     * ComputeTabPosition pass 5:
+     * Computes the clipping, visibility and adjust overall alpha if needed.
+     */
+    private void computeTabClippingVisibilityHelper() {
+        // alpha override, clipping and culling.
+        final boolean portrait = mCurrentMode == Orientation.PORTRAIT;
+
+        // Iterate through each tab starting at the top of the stack and working
+        // backwards. Set the clip on each tab such that it does not extend past
+        // the beginning of the tab above it. clipOffset is used to keep track
+        // of where the previous tab started.
+        float clipOffset;
+        if (portrait) {
+            // portrait LTR & RTL
+            clipOffset = mLayout.getHeight() + StackTab.sStackedTabVisibleSize;
+        } else if (!LocalizationUtils.isLayoutRtl()) {
+            // landscape LTR
+            clipOffset = mLayout.getWidth() + StackTab.sStackedTabVisibleSize;
+        } else {
+            // landscape RTL
+            clipOffset = -StackTab.sStackedTabVisibleSize;
+        }
+
+        for (int i = mStackTabs.length - 1; i >= 0; i--) {
+            LayoutTab layoutTab = mStackTabs[i].getLayoutTab();
+            layoutTab.setVisible(true);
+
+            // Don't bother with clipping tabs that are dying, rotating, with an X offset, or
+            // non-opaque.
+            if (mStackTabs[i].isDying() || mStackTabs[i].getXInStackOffset() != 0.0f
+                    || layoutTab.getAlpha() < 1.0f) {
+                layoutTab.setClipOffset(0.0f, 0.0f);
+                layoutTab.setClipSize(Float.MAX_VALUE, Float.MAX_VALUE);
+                continue;
+            }
+
+            // The beginning, size, and clipped size of the current tab.
+            float tabOffset, tabSize, tabClippedSize, borderAdjustmentSize, insetBorderPadding;
+            if (portrait) {
+                // portrait LTR & RTL
+                tabOffset = layoutTab.getY();
+                tabSize = layoutTab.getScaledContentHeight();
+                tabClippedSize = Math.min(tabSize, clipOffset - tabOffset);
+                borderAdjustmentSize = mBorderTransparentTop;
+                insetBorderPadding = mBorderTopPadding;
+            } else if (!LocalizationUtils.isLayoutRtl()) {
+                // landscape LTR
+                tabOffset = layoutTab.getX();
+                tabSize = layoutTab.getScaledContentWidth();
+                tabClippedSize = Math.min(tabSize, clipOffset - tabOffset);
+                borderAdjustmentSize = mBorderTransparentSide;
+                insetBorderPadding = 0;
+            } else {
+                // landscape RTL
+                tabOffset = layoutTab.getX() + layoutTab.getScaledContentWidth();
+                tabSize = layoutTab.getScaledContentWidth();
+                tabClippedSize = Math.min(tabSize, tabOffset - clipOffset);
+                borderAdjustmentSize = -mBorderTransparentSide;
+                insetBorderPadding = 0;
+            }
+
+            float absBorderAdjustmentSize = Math.abs(borderAdjustmentSize);
+
+            if (tabClippedSize <= absBorderAdjustmentSize) {
+                // If the tab is completed covered, don't bother drawing it at all.
+                layoutTab.setVisible(false);
+                layoutTab.setDrawDecoration(true);
+            } else {
+                // Fade the tab as it gets too close to the next one. This helps
+                // prevent overlapping shadows from becoming too dark.
+                float fade = MathUtils.clamp(((tabClippedSize - absBorderAdjustmentSize)
+                                                     / StackTab.sStackedTabVisibleSize),
+                        0, 1);
+                layoutTab.setDecorationAlpha(fade);
+
+                // When tabs tilt forward, it will expose more of the tab
+                // underneath. To compensate, make the clipping size larger.
+                // Note, this calculation is only an estimate that seems to
+                // work.
+                float clipScale = 1.0f;
+                if (layoutTab.getTiltX() > 0 || ((!portrait && LocalizationUtils.isLayoutRtl())
+                                                                ? layoutTab.getTiltY() < 0
+                                                                : layoutTab.getTiltY() > 0)) {
+                    final float tilt =
+                            Math.max(layoutTab.getTiltX(), Math.abs(layoutTab.getTiltY()));
+                    clipScale += (tilt / mMaxOverScrollAngle) * 0.60f;
+                }
+
+                float scaledTabClippedSize = Math.min(tabClippedSize * clipScale, tabSize);
+                // Set the clip
+                layoutTab.setClipOffset((!portrait && LocalizationUtils.isLayoutRtl())
+                                ? (tabSize - scaledTabClippedSize)
+                                : 0,
+                        0);
+                layoutTab.setClipSize(portrait ? Float.MAX_VALUE : scaledTabClippedSize,
+                        portrait ? scaledTabClippedSize : Float.MAX_VALUE);
+            }
+
+            // Clip the next tab where this tab begins.
+            if (i > 0) {
+                LayoutTab nextLayoutTab = mStackTabs[i - 1].getLayoutTab();
+                if (nextLayoutTab.getScale() <= layoutTab.getScale()) {
+                    clipOffset = tabOffset;
+                } else {
+                    clipOffset = tabOffset + tabClippedSize * layoutTab.getScale();
+                }
+
+                // Extend the border just a little bit. Otherwise, the
+                // rounded borders will intersect and make it look like the
+                // content is actually smaller.
+                clipOffset += borderAdjustmentSize;
+
+                if (layoutTab.getBorderAlpha() < 1.f && layoutTab.getToolbarAlpha() < 1.f) {
+                    clipOffset += insetBorderPadding;
+                }
+            }
+        }
+    }
+
+    /**
+     * ComputeTabPosition pass 6:
+     * Updates the visibility sorting value to use to figure out which thumbnails to load.
+     *
+     * @param stackRect The frame of the stack.
+     */
+    private void computeTabVisibilitySortingHelper(RectF stackRect) {
+        int referenceIndex = mReferenceOrderIndex;
+        if (referenceIndex == -1) {
+            int centerIndex =
+                    getTabIndexAtPositon(mLayout.getWidth() / 2.0f, mLayout.getHeight() / 2.0f);
+            // Alter the center to take into account the scrolling direction.
+            if (mCurrentScrollDirection > 0) centerIndex++;
+            if (mCurrentScrollDirection < 0) centerIndex--;
+            referenceIndex = MathUtils.clamp(centerIndex, 0, mStackTabs.length - 1);
+        }
+
+        final float width = mLayout.getWidth();
+        final float height = mLayout.getHeight();
+        final float left = MathUtils.clamp(stackRect.left, 0, width);
+        final float right = MathUtils.clamp(stackRect.right, 0, width);
+        final float top = MathUtils.clamp(stackRect.top, 0, height);
+        final float bottom = MathUtils.clamp(stackRect.bottom, 0, height);
+        final float stackArea = (right - left) * (bottom - top);
+        final float layoutArea = Math.max(width * height, 1.0f);
+        final float stackVisibilityMultiplier = stackArea / layoutArea;
+
+        for (int i = 0; i < mStackTabs.length; i++) {
+            mStackTabs[i].updateStackVisiblityValue(stackVisibilityMultiplier);
+            mStackTabs[i].updateVisiblityValue(referenceIndex);
+        }
+    }
+
+    /**
+     * Determine the current amount of overscroll. If the value is 0, there is
+     * no overscroll. If the value is < 0, tabs are overscrolling towards the
+     * top or or left. If the value is > 0, tabs are overscrolling towards the
+     * bottom or right.
+     */
+    private float computeOverscrollPercent() {
+        if (mOverScrollOffset >= 0) {
+            return mOverScrollOffset / mMaxOverScroll;
+        } else {
+            return mOverScrollOffset / mMaxUnderScroll;
+        }
+    }
+
+    /**
+     * ComputeTabPosition pass 4:
+     * Update the tilt of each tab.
+     *
+     * @param time      The current time of the app in ms.
+     * @param stackRect The frame of the stack.
+     */
+    private void computeTabTiltHelper(long time, RectF stackRect) {
+        final boolean portrait = mCurrentMode == Orientation.PORTRAIT;
+        final float parentWidth = stackRect.width();
+        final float parentHeight = stackRect.height();
+        final float overscrollPercent = computeOverscrollPercent();
+
+        // All the animations that sets the tilt value must be listed here.
+        if (mOverviewAnimationType == OverviewAnimationType.START_PINCH
+                || mOverviewAnimationType == OverviewAnimationType.DISCARD
+                || mOverviewAnimationType == OverviewAnimationType.FULL_ROLL
+                || mOverviewAnimationType == OverviewAnimationType.TAB_FOCUSED
+                || mOverviewAnimationType == OverviewAnimationType.UNDISCARD
+                || mOverviewAnimationType == OverviewAnimationType.DISCARD_ALL) {
+            // Let the animation handle setting tilt values
+        } else if (mPinch0TabIndex >= 0 || overscrollPercent == 0.0f
+                || mOverviewAnimationType == OverviewAnimationType.REACH_TOP) {
+            // Keep tabs flat during pinch
+            for (int i = 0; i < mStackTabs.length; ++i) {
+                StackTab stackTab = mStackTabs[i];
+                LayoutTab layoutTab = stackTab.getLayoutTab();
+                layoutTab.setTiltX(0, 0);
+                layoutTab.setTiltY(0, 0);
+            }
+        } else if (overscrollPercent < 0) {
+            if (mOverScrollCounter >= OVERSCROLL_FULL_ROLL_TRIGGER) {
+                startAnimation(time, OverviewAnimationType.FULL_ROLL);
+                mOverScrollCounter = 0;
+                // Remove overscroll so when the animation finishes the overscroll won't
+                // be bothering.
+                setScrollTarget(
+                        MathUtils.clamp(mScrollOffset, getMinScroll(false), getMaxScroll(false)),
+                        false);
+            } else {
+                // Handle tilting tabs backwards (top or left of the tab goes away
+                // from the camera). Each tab pivots the same amount around the
+                // same point on the screen. The pivot point is the middle of the
+                // top tab.
+
+                float tilt = 0;
+                if (overscrollPercent < -OVERSCROLL_TOP_SLIDE_PCTG) {
+                    // Start tilting tabs after they're done sliding together.
+                    float scaledOverscroll = (overscrollPercent + OVERSCROLL_TOP_SLIDE_PCTG)
+                            / (1 - OVERSCROLL_TOP_SLIDE_PCTG);
+                    tilt = mUnderScrollAngleInterpolator.getInterpolation(-scaledOverscroll)
+                            * -mMaxOverScrollAngle * BACKWARDS_TILT_SCALE;
+                }
+
+                float pivotOffset = 0;
+                LayoutTab topTab = mStackTabs[mStackTabs.length - 1].getLayoutTab();
+                pivotOffset = portrait ? topTab.getScaledContentHeight() / 2 + topTab.getY()
+                                       : topTab.getScaledContentWidth() / 2 + topTab.getX();
+
+                for (int i = 0; i < mStackTabs.length; ++i) {
+                    StackTab stackTab = mStackTabs[i];
+                    LayoutTab layoutTab = stackTab.getLayoutTab();
+                    if (portrait) {
+                        layoutTab.setTiltX(tilt, pivotOffset - layoutTab.getY());
+                    } else {
+                        layoutTab.setTiltY(LocalizationUtils.isLayoutRtl() ? -tilt : tilt,
+                                pivotOffset - layoutTab.getX());
+                    }
+                }
+            }
+        } else {
+            // Handle tilting tabs forwards (top or left of the tab comes
+            // towards the camera). Each tab pivots around a point 1/3 of the
+            // way down from the top/left of itself. The angle angle is scaled
+            // based on its distance away from the top/left.
+
+            float tilt = mOverScrollAngleInterpolator.getInterpolation(overscrollPercent)
+                    * mMaxOverScrollAngle;
+            float offset = mOverscrollSlideInterpolator.getInterpolation(overscrollPercent)
+                    * mMaxOverScrollSlide;
+
+            for (int i = 0; i < mStackTabs.length; ++i) {
+                StackTab stackTab = mStackTabs[i];
+                LayoutTab layoutTab = stackTab.getLayoutTab();
+                if (portrait) {
+                    // portrait LTR & RTL
+                    float adjust = MathUtils.clamp((layoutTab.getY() / parentHeight) + 0.50f, 0, 1);
+                    layoutTab.setTiltX(tilt * adjust, layoutTab.getScaledContentHeight() / 3);
+                    layoutTab.setY(layoutTab.getY() + offset);
+                } else if (LocalizationUtils.isLayoutRtl()) {
+                    // landscape RTL
+                    float adjust = MathUtils.clamp(-(layoutTab.getX() / parentWidth) + 0.50f, 0, 1);
+                    layoutTab.setTiltY(-tilt * adjust, layoutTab.getScaledContentWidth() * 2 / 3);
+                    layoutTab.setX(layoutTab.getX() - offset);
+                } else {
+                    // landscape LTR
+                    float adjust = MathUtils.clamp((layoutTab.getX() / parentWidth) + 0.50f, 0, 1);
+                    layoutTab.setTiltY(tilt * adjust, layoutTab.getScaledContentWidth() / 3);
+                    layoutTab.setX(layoutTab.getX() + offset);
+                }
+            }
+        }
+    }
+
+    /**
+     * Computes the {@link LayoutTab} position from the stack and the stackTab data.
+     *
+     * @param time      The current time of the app in ms.
+     * @param stackRect The rectangle the stack should be drawn into. It may change over frames.
+     */
+    public void computeTabPosition(long time, RectF stackRect) {
+        if (mStackTabs == null || mStackTabs.length == 0) return;
+
+        if (!mRecomputePosition) return;
+        mRecomputePosition = false;
+
+        // Step 1: Updates the {@link LayoutTab} scale, alpha and depth values.
+        computeTabScaleAlphaDepthHelper(stackRect);
+
+        // Step 2: Fix tab scroll offsets to avoid gaps.
+        computeTabScrollOffsetHelper();
+
+        // Step 3: Compute the actual position.
+        computeTabOffsetHelper(stackRect);
+
+        // Step 4: Update the tilt of each tab.
+        computeTabTiltHelper(time, stackRect);
+
+        // Step 5: Clipping, visibility and adjust overall alpha.
+        computeTabClippingVisibilityHelper();
+
+        // Step 6: Update visibility sorting for prioritizing thumbnail texture request.
+        computeTabVisibilitySortingHelper(stackRect);
+    }
+
+    /**
+     * @param stackFocus The current amount of focus of the stack [0 .. 1]
+     * @param orderIndex The index in the stack of the focused tab. -1 to ask the
+     *                   stack to compute it.
+     */
+    public void setStackFocusInfo(float stackFocus, int orderIndex) {
+        if (mStackTabs == null) return;
+        mReferenceOrderIndex = orderIndex;
+        for (int i = 0; i < mStackTabs.length; i++) {
+            mStackTabs[i].getLayoutTab().setBorderCloseButtonAlpha(stackFocus);
+        }
+    }
+
+    /**
+     * Reverts the closure of the tab specified by {@code tabId}.  This will run an undiscard
+     * animation on that tab.
+     * @param time  The current time of the app in ms.
+     * @param tabId The id of the tab to animate.
+     */
+    public void undoClosure(long time, int tabId) {
+        createStackTabs(true);
+        if (mStackTabs == null) return;
+
+        for (int i = 0; i < mStackTabs.length; i++) {
+            StackTab tab = mStackTabs[i];
+
+            if (tab.getId() == tabId) {
+                tab.setDiscardAmount(getDiscardRange());
+                tab.setDying(false);
+                tab.getLayoutTab().setMaxContentHeight(mLayout.getHeightMinusTopControls());
+            }
+        }
+
+        mSpacing = computeSpacing(mStackTabs.length);
+        startAnimation(time, OverviewAnimationType.UNDISCARD);
+    }
+
+    /**
+     * Creates the {@link StackTab}s needed for display and populates {@link #mStackTabs}.
+     * It is called from show() at the beginning of every new draw phase. It tries to reuse old
+     * {@link StackTab} instead of creating new ones every time.
+     * @param restoreState Whether or not to restore the {@link LayoutTab} state when we rebuild the
+     *                     {@link StackTab}s.  There are some properties like maximum content size
+     *                     or whether or not to show the toolbar that might have to be restored if
+     *                     we're calling this while the switcher is already visible.
+     */
+    private void createStackTabs(boolean restoreState) {
+        final int count = mTabModel.getCount();
+        if (count == 0) {
+            cleanupTabs();
+        } else {
+            StackTab[] oldTabs = mStackTabs;
+            mStackTabs = new StackTab[count];
+
+            final boolean isIncognito = mTabModel.isIncognito();
+            final boolean needTitle = !mLayout.isHiding();
+            for (int i = 0; i < count; ++i) {
+                Tab tab = mTabModel.getTabAt(i);
+                int tabId = tab != null ? tab.getId() : Tab.INVALID_TAB_ID;
+                mStackTabs[i] = findTabById(oldTabs, tabId);
+
+                float maxContentWidth = -1.f;
+                float maxContentHeight = -1.f;
+
+                if (mStackTabs[i] != null && mStackTabs[i].getLayoutTab() != null && restoreState) {
+                    maxContentWidth = mStackTabs[i].getLayoutTab().getMaxContentWidth();
+                    maxContentHeight = mStackTabs[i].getLayoutTab().getMaxContentHeight();
+                }
+
+                LayoutTab layoutTab = mLayout.createLayoutTab(tabId, isIncognito,
+                        Layout.SHOW_CLOSE_BUTTON, needTitle, maxContentWidth, maxContentHeight);
+                layoutTab.setInsetBorderVertical(true);
+                layoutTab.setShowToolbar(true);
+                layoutTab.setToolbarAlpha(0.f);
+                layoutTab.setAnonymizeToolbar(mTabModel.index() != i);
+
+                if (mStackTabs[i] == null) {
+                    mStackTabs[i] = new StackTab(layoutTab);
+                } else {
+                    mStackTabs[i].setLayoutTab(layoutTab);
+                }
+
+                mStackTabs[i].setNewIndex(i);
+                // The initial enterStack animation will take care of
+                // positioning, scaling, etc.
+            }
+        }
+    }
+
+    private StackTab findTabById(StackTab[] layoutTabs, int id) {
+        if (layoutTabs == null) return null;
+        final int count = layoutTabs.length;
+        for (int i = 0; i < count; i++) {
+            if (layoutTabs[i].getId() == id) return layoutTabs[i];
+        }
+        return null;
+    }
+
+    /**
+     * Creates a {@link StackTab}.
+     * This function should ONLY be called from {@link #tabCreated(long, int)} and nowhere else.
+     *
+     * @param id The id of the tab.
+     * @return   Whether the tab has successfully been created and added.
+     */
+    private boolean createTabHelper(int id) {
+        if (TabModelUtils.getTabById(mTabModel, id) == null) return false;
+
+        // Check to see if the tab already exists in our model.  This is
+        // just to cover the case where stackEntered and then tabCreated()
+        // called in a row.
+        if (mStackTabs != null) {
+            final int count = mStackTabs.length;
+            for (int i = 0; i < count; ++i) {
+                if (mStackTabs[i].getId() == id) {
+                    return false;
+                }
+            }
+        }
+
+        createStackTabs(true);
+
+        return true;
+    }
+
+    private int computeSpacing(int layoutTabCount) {
+        // This redetermines the proper spacing for the {@link StackTab}.  It takes in
+        // a parameter for the size instead of using the mStackTabs.length
+        // property because we could be setting the spacing for a delete
+        // before the tab has been removed (will help with animations).
+        int spacing = 0;
+        if (layoutTabCount > 1) {
+            final float dimension = getScrollDimensionSize();
+            int minSpacing = (int) Math.max(dimension * SPACING_SCREEN, mMinSpacing);
+            if (mStackTabs != null) {
+                for (int i = 0; i < mStackTabs.length; i++) {
+                    assert mStackTabs[i] != null;
+                    if (!mStackTabs[i].isDying()) {
+                        minSpacing = (int) Math.min(
+                                minSpacing, mStackTabs[i].getSizeInScrollDirection(mCurrentMode));
+                    }
+                }
+            }
+            spacing = (int) ((dimension - 20) / (layoutTabCount * .8f));
+            spacing = Math.max(spacing, minSpacing);
+        }
+        return spacing;
+    }
+
+    private float getStackScale(RectF stackRect) {
+        return mCurrentMode == Orientation.PORTRAIT
+                ? stackRect.width() / mLayout.getWidth()
+                : stackRect.height() / mLayout.getHeightMinusTopControls();
+    }
+
+    private void setScrollTarget(float offset, boolean immediate) {
+        // Ensure that the stack cannot be scrolled too far in either direction.
+        // mScrollOffset is clamped between [-min, 0], where offset 0 has the
+        // farthest back tab (the first tab) at the top, with everything else
+        // pulled down, and -min has the tab at the top of the stack (the last
+        // tab) is pulled up and fully visible.
+        final boolean overscroll = allowOverscroll();
+        mScrollTarget = MathUtils.clamp(offset, getMinScroll(overscroll), getMaxScroll(overscroll));
+        if (immediate) mScrollOffset = mScrollTarget;
+        mCurrentScrollDirection = Math.signum(mScrollTarget - mScrollOffset);
+    }
+
+    private float getMinScroll(boolean allowUnderScroll) {
+        float maxOffset = 0;
+        if (mStackTabs != null) {
+            // The tabs are not always ordered so we need to browse them all.
+            for (int i = 0; i < mStackTabs.length; i++) {
+                if (!mStackTabs[i].isDying() && mStackTabs[i].getLayoutTab().isVisible()) {
+                    maxOffset = Math.max(mStackTabs[i].getScrollOffset(), maxOffset);
+                }
+            }
+        }
+        return (allowUnderScroll ? -mMaxUnderScroll : 0) - maxOffset;
+    }
+
+    /**
+     * Gets the max scroll value.
+     *
+     * @param allowOverscroll True if overscroll is allowed.
+     */
+    private float getMaxScroll(boolean allowOverscroll) {
+        if (mStackTabs == null || !allowOverscroll) {
+            return 0;
+        } else {
+            return mMaxOverScroll;
+        }
+    }
+
+    private void stopScrollingMovement(long time) {
+        // We have to cancel the fling if it is in progress.
+        if (mScroller.computeScrollOffset(time)) {
+            // Set the current offset and target to the current scroll
+            // position so the {@link StackTab}s won't scroll anymore.
+            setScrollTarget(mScroller.getCurrY(), true /* immediate */);
+
+            // Tell the scroller to finish scrolling.
+            mScroller.forceFinished(true);
+        } else {
+            // If we aren't scrolling just set the target to the current
+            // offset so we don't move anymore.
+            setScrollTarget(mScrollOffset, false);
+        }
+    }
+
+    private boolean allowOverscroll() {
+        // All the animations that want to leave the tilt value to be set by the overscroll must
+        // be added here.
+        return (mOverviewAnimationType == OverviewAnimationType.NONE
+                       || mOverviewAnimationType == OverviewAnimationType.VIEW_MORE
+                       || mOverviewAnimationType == OverviewAnimationType.ENTER_STACK)
+                && mPinch0TabIndex < 0;
+    }
+
+    /**
+     * Smoothes input signal. The definition of the input is lower than the
+     * pixel density of the screen so we need to smooth the input to give the illusion of smooth
+     * animation on screen from chunky inputs.
+     * The combination of 20 pixels and 0.9f ensures that the output is not more than 2 pixels away
+     * from the target.
+     * TODO: This has nothing to do with time, just draw rate.
+     *       Is this okay or do we want to have the interpolation based on the time elapsed?
+     * @param current   The current value of the signal.
+     * @param input     The raw input value.
+     * @return          The smoothed signal.
+     */
+    private float smoothInput(float current, float input) {
+        current = MathUtils.clamp(current, input - 20, input + 20);
+        return MathUtils.interpolate(current, input, 0.9f);
+    }
+
+    private void forceScrollStop() {
+        mScroller.forceFinished(true);
+        updateOverscrollOffset();
+        mScrollTarget = mScrollOffset;
+    }
+
+    private void updateScrollOffset(long time) {
+        // If we are still scrolling, which is determined by a disparity
+        // between our scroll offset and our scroll target, we need
+        // to try to move closer to that position.
+        if (mScrollOffset != mScrollTarget) {
+            if (mScroller.computeScrollOffset(time)) {
+                final float newScrollOffset = mScroller.getCurrY();
+                evenOutTabs(newScrollOffset - mScrollOffset, true);
+                // We are currently in the process of being flinged.  Just
+                // ask the scroller for the new position.
+                mScrollOffset = newScrollOffset;
+            } else {
+                // We are just being dragged or scrolled, not flinged.  This
+                // means we should move closer to our target quickly but not
+                // quickly enough to show the stuttering that could be
+                // exposed by the touch event rate.
+                mScrollOffset = smoothInput(mScrollOffset, mScrollTarget);
+            }
+            requestUpdate();
+        } else {
+            // Make sure that the scroller is marked as finished when the destination is reached.
+            mScroller.forceFinished(true);
+        }
+        updateOverscrollOffset();
+    }
+
+    private void updateOverscrollOffset() {
+        float clamped = MathUtils.clamp(mScrollOffset, getMinScroll(false), getMaxScroll(false));
+        if (!allowOverscroll()) {
+            mScrollOffset = clamped;
+        }
+        float overscroll = mScrollOffset - clamped;
+
+        // Counts the number of overscroll push in the same direction in a row.
+        int derivativeState = (int) Math.signum(Math.abs(mOverScrollOffset) - Math.abs(overscroll));
+        if (derivativeState != mOverScrollDerivative && derivativeState == 1 && overscroll < 0) {
+            mOverScrollCounter++;
+        } else if (overscroll > 0 || mCurrentMode == Orientation.LANDSCAPE) {
+            mOverScrollCounter = 0;
+        }
+        mOverScrollDerivative = derivativeState;
+
+        mOverScrollOffset = overscroll;
+    }
+
+    private void resetAllScrollOffset() {
+        if (mTabModel == null) return;
+        // Reset the scroll position to put the important {@link StackTab} into focus.
+        // This does not scroll the {@link StackTab}s there but rather moves everything
+        // there immediately.
+        // The selected tab is supposed to show at the center of the screen.
+        float maxTabsPerPage = getScrollDimensionSize() / mSpacing;
+        float centerOffsetIndex = maxTabsPerPage / 2.0f - 0.5f;
+        final int count = mTabModel.getCount();
+        final int index = mTabModel.index();
+        if (index < centerOffsetIndex || count <= maxTabsPerPage) {
+            mScrollOffset = 0;
+        } else if (index == count - 1 && Math.ceil(maxTabsPerPage) < count) {
+            mScrollOffset = (maxTabsPerPage - count - 1) * mSpacing;
+        } else if ((count - index - 1) < centerOffsetIndex) {
+            mScrollOffset = (maxTabsPerPage - count) * mSpacing;
+        } else {
+            mScrollOffset = (centerOffsetIndex - index) * mSpacing;
+        }
+        // Reset the scroll offset of the tabs too.
+        if (mStackTabs != null) {
+            for (int i = 0; i < mStackTabs.length; i++) {
+                mStackTabs[i].setScrollOffset(screenToScroll(i * mSpacing));
+            }
+        }
+        setScrollTarget(mScrollOffset, false);
+    }
+
+    private float approxScreen(StackTab tab, float globalScrollOffset) {
+        return StackTab.scrollToScreen(tab.getScrollOffset() + globalScrollOffset, mWarpSize);
+    }
+
+    private float scrollToScreen(float scrollSpace) {
+        return StackTab.scrollToScreen(scrollSpace, mWarpSize);
+    }
+
+    private float screenToScroll(float screenSpace) {
+        return StackTab.screenToScroll(screenSpace, mWarpSize);
+    }
+
+    /**
+     * @return The range of the discard action. At the end of the +/- range the discarded tab
+     *         will be fully transparent.
+     */
+    private float getDiscardRange() {
+        return getRange(DISCARD_RANGE_SCREEN);
+    }
+
+    private float getRange(float range) {
+        return range * (mCurrentMode == Orientation.PORTRAIT ? mLayout.getWidth()
+                                                             : mLayout.getHeightMinusTopControls());
+    }
+
+    /**
+     * Computes the scale of the tab based on its discard status.
+     *
+     * @param amount    The discard amount.
+     * @param range     The range of the absolute value of discard amount.
+     * @param fromClick Whether or not the discard was from a click or a swipe.
+     * @return          The scale of the tab to use to draw the tab.
+     */
+    public static float computeDiscardScale(float amount, float range, boolean fromClick) {
+        if (Math.abs(amount) < 1.0f) return 1.0f;
+        float t = amount / range;
+        float endScale = fromClick ? DISCARD_END_SCALE_CLICK : DISCARD_END_SCALE_SWIPE;
+        return MathUtils.interpolate(1.0f, endScale, Math.abs(t));
+    }
+
+    /**
+     * Computes the alpha value of the tab based on its discard status.
+     *
+     * @param amount The discard amount.
+     * @param range  The range of the absolute value of discard amount.
+     * @return       The alpha value that need to be applied on the tab.
+     */
+    public static float computeDiscardAlpha(float amount, float range) {
+        if (Math.abs(amount) < 1.0f) return 1.0f;
+        float t = amount / range;
+        t = MathUtils.clamp(t, -1.0f, 1.0f);
+        return 1.f - Math.abs(t);
+    }
+
+    private void updateCurrentMode(int orientation) {
+        mCurrentMode = orientation;
+        mDiscardDirection = getDefaultDiscardDirection();
+        setWarpState(true, false);
+        final float opaqueTopPadding = mBorderTopPadding - mBorderTransparentTop;
+        mAnimationFactory = StackAnimation.createAnimationFactory(mLayout.getWidth(),
+                mLayout.getHeight(), mLayout.getHeightMinusTopControls(), mBorderTopPadding,
+                opaqueTopPadding, mBorderLeftPadding, mCurrentMode);
+        float dpToPx = mLayout.getContext().getResources().getDisplayMetrics().density;
+        mViewAnimationFactory = new StackViewAnimation(dpToPx, mLayout.getWidth());
+        if (mStackTabs == null) return;
+        float width = mLayout.getWidth();
+        float height = mLayout.getHeightMinusTopControls();
+        for (int i = 0; i < mStackTabs.length; i++) {
+            LayoutTab tab = mStackTabs[i].getLayoutTab();
+            if (tab == null) continue;
+            tab.setMaxContentWidth(width);
+            tab.setMaxContentHeight(height);
+        }
+    }
+
+    /**
+     * Called to release everything. Called well after the view has been really hidden.
+     */
+    public void cleanupTabs() {
+        mStackTabs = null;
+        resetInputActionIndices();
+    }
+
+    /**
+     * Resets all the indices that are pointing to tabs for various features.
+     */
+    private void resetInputActionIndices() {
+        mPinch0TabIndex = -1;
+        mPinch1TabIndex = -1;
+        mScrollingTab = null;
+        mDiscardingTab = null;
+        mLongPressSelected = -1;
+    }
+
+    /**
+     * Invalidates the current graphics and force to recomputes tab placements.
+     */
+    public void requestUpdate() {
+        mRecomputePosition = true;
+        mLayout.requestUpdate();
+    }
+
+    /**
+     * Reset session based parameters.
+     * Called before the a session starts. Before the show, regardless if the stack is displayable.
+     */
+    public void reset() {
+        mIsDying = false;
+    }
+
+    /**
+     * Whether or not the tab positions warp from linear to nonlinear as the tabs approach the edge
+     * of the screen.  This allows us to move the tabs to linear space to track finger movements,
+     * but also move them back to non-linear space without any visible change to the user.
+     * @param canWarp           Whether or not the tabs are allowed to warp.
+     * @param adjustCurrentTabs Whether or not to change the tab positions so there's no visible
+     *                          difference after the change.
+     */
+    private void setWarpState(boolean canWarp, boolean adjustCurrentTabs) {
+        float warp = canWarp ? getScrollDimensionSize() * SCROLL_WARP_PCTG : 0.f;
+
+        if (mStackTabs != null && adjustCurrentTabs && Float.compare(warp, mWarpSize) != 0) {
+            float scrollOffset =
+                    MathUtils.clamp(mScrollOffset, getMinScroll(false), getMaxScroll(false));
+            for (int i = 0; i < mStackTabs.length; i++) {
+                StackTab tab = mStackTabs[i];
+                float tabScrollOffset = tab.getScrollOffset();
+                float tabScrollSpace = tabScrollOffset + scrollOffset;
+                float tabScreen = StackTab.scrollToScreen(tabScrollSpace, mWarpSize);
+                float tabScrollSpaceFinal = StackTab.screenToScroll(tabScreen, warp);
+                float scrollDelta = tabScrollSpaceFinal - tabScrollSpace;
+                tab.setScrollOffset(tabScrollOffset + scrollDelta);
+            }
+        }
+
+        mWarpSize = warp;
+    }
+
+    /**
+     * Called when the swipe animation get initiated. It gives a chance to initialize everything.
+     * @param time      The current time of the app in ms.
+     * @param direction The direction the swipe is in.
+     * @param x         The horizontal coordinate the swipe started at in dp.
+     * @param y         The vertical coordinate the swipe started at in dp.
+     */
+    public void swipeStarted(long time, ScrollDirection direction, float x, float y) {
+        if (direction != ScrollDirection.DOWN) return;
+
+        // Turn off warping the tabs because we need them to track the user's finger.
+        setWarpState(false, false);
+
+        // Restart the enter stack animation with the new warp values.
+        startAnimation(time, OverviewAnimationType.ENTER_STACK);
+
+        // Update the scroll offset to put the focused tab at the top.
+        final int index = mTabModel.index();
+
+        if (mCurrentMode == Orientation.PORTRAIT) {
+            mScrollOffset = -index * mSpacing;
+        } else {
+            mScrollOffset = -index * mSpacing + x - LANDSCAPE_SWIPE_DRAG_TAB_OFFSET_DP;
+            mScrollOffset =
+                    MathUtils.clamp(mScrollOffset, getMinScroll(false), getMaxScroll(false));
+        }
+        setScrollTarget(mScrollOffset, true);
+
+        // Don't let the tabs even out during this scroll.
+        mEvenOutProgress = 1.f;
+
+        // Set up the tracking scroll parameters.
+        mSwipeUnboundScrollOffset = mScrollOffset;
+        mSwipeBoundedScrollOffset = mScrollOffset;
+
+        // Reset other state.
+        mSwipeIsCancelable = false;
+        mSwipeCanScroll = false;
+        mInSwipe = true;
+    }
+
+    /**
+     * Updates a swipe gesture.
+     * @param time The current time of the app in ms.
+     * @param x    The horizontal coordinate the swipe is currently at in dp.
+     * @param y    The vertical coordinate the swipe is currently at in dp.
+     * @param dx   The horizontal delta since the last update in dp.
+     * @param dy   The vertical delta since the last update in dp.
+     * @param tx   The horizontal difference between the start and the current position in dp.
+     * @param ty   The vertical difference between the start and the current position in dp.
+     */
+    public void swipeUpdated(long time, float x, float y, float dx, float dy, float tx, float ty) {
+        if (!mInSwipe) return;
+
+        final float toolbarSize = mLayout.getHeight() - mLayout.getHeightMinusTopControls();
+        if (ty > toolbarSize) mSwipeCanScroll = true;
+        if (!mSwipeCanScroll) return;
+
+        final int index = mTabModel.index();
+
+        // Check to make sure the index is still valid.
+        if (index < 0 || index >= mStackTabs.length) {
+            assert false : "Tab index out of bounds in Stack#swipeUpdated()";
+            return;
+        }
+
+        final float delta = mCurrentMode == Orientation.PORTRAIT ? dy : dx;
+
+        // Update the unbound scroll offset, tracking delta regardless of constraints.
+        mSwipeUnboundScrollOffset += delta;
+
+        // Figure out the new constrained position.
+        final float minScroll = getMinScroll(true);
+        final float maxScroll = getMaxScroll(true);
+        float offset = MathUtils.clamp(mSwipeUnboundScrollOffset, minScroll, maxScroll);
+
+        final float constrainedDelta = offset - mSwipeBoundedScrollOffset;
+        mSwipeBoundedScrollOffset = offset;
+
+        if (constrainedDelta == 0.f) return;
+
+        if (mCurrentMode == Orientation.PORTRAIT) {
+            dy = constrainedDelta;
+        } else {
+            dx = constrainedDelta;
+        }
+
+        // Propagate the new drag event.
+        drag(time, x, y, dx, dy);
+
+        // Figure out if the user has scrolled down enough that they can scroll back up and exit.
+        if (mCurrentMode == Orientation.PORTRAIT) {
+            // The cancelable threshold is determined by the top position of the tab in the stack.
+            final float discardOffset = mStackTabs[index].getScrollOffset();
+            final boolean beyondThreshold = -mScrollOffset < discardOffset;
+
+            // Allow the user to cancel in the future if they're beyond the threshold.
+            mSwipeIsCancelable |= beyondThreshold;
+
+            // If the user can cancel the swipe and they're back behind the threshold, cancel.
+            if (mSwipeIsCancelable && !beyondThreshold) swipeCancelled(time);
+        } else {
+            // The cancelable threshold is determined by the top position of the tab.
+            final float discardOffset = mStackTabs[index].getLayoutTab().getY();
+
+            boolean aboveThreshold = discardOffset < getRange(SWIPE_LANDSCAPE_THRESHOLD);
+
+            mSwipeIsCancelable |= !aboveThreshold;
+
+            if (mSwipeIsCancelable && aboveThreshold) swipeCancelled(time);
+        }
+    }
+
+    /**
+     * Called when the swipe ends; most likely on finger up event. It gives a chance to start
+     * an ending animation to exit the mode gracefully.
+     * @param time The current time of the app in ms.
+     */
+    public void swipeFinished(long time) {
+        if (!mInSwipe) return;
+
+        mInSwipe = false;
+
+        // Reset the warp state and mark the tabs to even themselves out.
+        setWarpState(true, true);
+        mEvenOutProgress = 0.f;
+
+        onUpOrCancel(time);
+    }
+
+    /**
+     * Called when the user has cancelled a swipe; most likely if they have dragged their finger
+     * back to the starting position.  Some handlers will throw swipeFinished() instead.
+     * @param time The current time of the app in ms.
+     */
+    public void swipeCancelled(long time) {
+        if (!mInSwipe) return;
+
+        mDiscardingTab = null;
+
+        mInSwipe = false;
+
+        setWarpState(true, true);
+        mEvenOutProgress = 0.f;
+
+        // Select the current tab so we exit the switcher.
+        Tab tab = TabModelUtils.getCurrentTab(mTabModel);
+        mLayout.uiSelectingTab(time, tab != null ? tab.getId() : Tab.INVALID_TAB_ID);
+    }
+
+    /**
+     * Fling from a swipe gesture.
+     * @param time The current time of the app in ms.
+     * @param x    The horizontal coordinate the swipe is currently at in dp.
+     * @param y    The vertical coordinate the swipe is currently at in dp.
+     * @param tx   The horizontal difference between the start and the current position in dp.
+     * @param ty   The vertical difference between the start and the current position in dp.
+     * @param vx   The horizontal velocity of the fling.
+     * @param vy   The vertical velocity of the fling.
+     */
+    public void swipeFlingOccurred(
+            long time, float x, float y, float tx, float ty, float vx, float vy) {
+        if (!mInSwipe) return;
+
+        // Propagate the fling data.
+        fling(time, x, y, vx, vy);
+
+        onUpOrCancel(time);
+    }
+}