Use a scrollable tab strip on tablets at small widths

chrome/android/java/src/org/chromium/chrome/browser/compositor/overlays/strip/ScrollingStripStacker.java

Index: chrome/android/java/src/org/chromium/chrome/browser/compositor/overlays/strip/ScrollingStripStacker.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/compositor/overlays/strip/ScrollingStripStacker.java b/chrome/android/java/src/org/chromium/chrome/browser/compositor/overlays/strip/ScrollingStripStacker.java
new file mode 100644
index 0000000000000000000000000000000000000000..f356bb0d964b8052f13e04ee051c420f2a127fbb
--- /dev/null
+++ b/chrome/android/java/src/org/chromium/chrome/browser/compositor/overlays/strip/ScrollingStripStacker.java
@@ -0,0 +1,99 @@
+// Copyright 2016 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.overlays.strip;
+
+import org.chromium.ui.base.LocalizationUtils;
+
+/**
+ * A stacker that tells the {@link StripLayoutHelper} how to layer the tabs for the
+ * {@link StaticLayout} when the available window width is < 600dp. Tabs will be stacked side by
+ * side and the entire strip will scroll. Tabs will never completely overlap each other.
+ */
+public class ScrollingStripStacker extends StripStacker {
+    @Override
+    public void setTabOffsets(int selectedIndex, StripLayoutTab[] indexOrderedTabs,
+            float tabStackWidth, int maxTabsToStack, float tabOverlapWidth, float stripLeftMargin,
+            float stripRightMargin, float stripWidth, boolean inReorderMode) {
+        for (int i = 0; i < indexOrderedTabs.length; i++) {
+            StripLayoutTab tab = indexOrderedTabs[i];
+            tab.setDrawX(tab.getIdealX());
+            tab.setDrawY(tab.getOffsetY());
+            tab.setVisiblePercentage(1.f);
+            tab.setContentOffsetX(0.f);
+        }
+    }
+
+    @Override
+    public float computeNewTabButtonOffset(StripLayoutTab[] indexOrderedTabs,
+            float tabOverlapWidth, float stripLeftMargin, float stripRightMargin, float stripWidth,
+            float newTabButtonWidth) {
+        return LocalizationUtils.isLayoutRtl()
+                ? computeNewTabButtonOffsetRtl(indexOrderedTabs, tabOverlapWidth, stripRightMargin,
+                        stripWidth, newTabButtonWidth)
+                                : computeNewTabButtonOffsetLtr(indexOrderedTabs, tabOverlapWidth,
+                                        stripLeftMargin, stripWidth);
+    }
+
+    @Override
+    public void performOcclusionPass(int selectedIndex, StripLayoutTab[] indexOrderedTabs,
+            float layoutWidth) {
+        for (int i = 0; i < indexOrderedTabs.length; i++) {
+            StripLayoutTab tab = indexOrderedTabs[i];
+            tab.setVisible((tab.getDrawX() + tab.getWidth()) >= 0 && tab.getDrawX() <= layoutWidth);
+        }
+    }
+
+    private float computeNewTabButtonOffsetLtr(StripLayoutTab[] indexOrderedTabs,
+            float tabOverlapWidth, float stripLeftMargin, float stripWidth) {
+        float rightEdge = stripLeftMargin;
+
+        int numTabs = indexOrderedTabs.length;
+        // Need to look at the last two tabs to determine the new tab position in case the last
+        // tab is dying.
+        int i = numTabs > 0 ? ((numTabs >= 2) ? numTabs - 2 : numTabs - 1) : 0;
+        for (; i < numTabs; i++) {
+            StripLayoutTab tab = indexOrderedTabs[i];
+            float layoutWidth;
+            if (tab.isDying()) {
+                // If a tab is dying, adjust the tab width by the width weight so that the new
+                // tab button slides to the left with the closing tab.
+                layoutWidth = tab.getWidth() * tab.getWidthWeight();
+            } else {
+                // If a tab is being created, disregard its width weight so the new tab button
+                // doesn't end up positioned too far to the left. If a tab is neither being
+                // created or dying, its width width weight is 1.0 and can also be ignored.
+                layoutWidth = tab.getWidth();
+            }
+
+            rightEdge = Math.max(tab.getDrawX() + layoutWidth, rightEdge);
+        }
+
+        // Adjust the right edge by the tab overlap width so that the new tab button is nestled
+        // closer to the tab.
+        rightEdge -= tabOverlapWidth / 2;
+
+        // The draw X position for the new tab button is the rightEdge of the tab strip.
+        return rightEdge;
+    }
+
+    private float computeNewTabButtonOffsetRtl(StripLayoutTab[] indexOrderedTabs,
+            float tabOverlapWidth, float stripRightMargin, float stripWidth,
+            float newTabButtonWidth) {
+        float leftEdge = stripWidth - stripRightMargin;
+        int numTabs = indexOrderedTabs.length;
+        if (numTabs >= 1) {
+            StripLayoutTab tab = indexOrderedTabs[numTabs - 1];
+            leftEdge = Math.min(tab.getDrawX(), leftEdge);
+        }
+
+        // Adjust the left edge by the tab overlap width so that the new tab button is nestled
+        // closer to the tab.
+        leftEdge += tabOverlapWidth / 2;
+
+        // The draw X position for the new tab button is the left edge of the tab strip minus
+        // the new tab button width.
+        return leftEdge - newTabButtonWidth;
+    }
+}