Add ScrollManager class

third_party/WebKit/Source/core/input/ScrollManager.cpp

Index: third_party/WebKit/Source/core/input/ScrollManager.cpp
diff --git a/third_party/WebKit/Source/core/input/ScrollManager.cpp b/third_party/WebKit/Source/core/input/ScrollManager.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..c6971393ecdca49667bbfb91f0cc808454d91103
--- /dev/null
+++ b/third_party/WebKit/Source/core/input/ScrollManager.cpp
@@ -0,0 +1,657 @@
+// 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.
+
+#include "core/input/ScrollManager.h"
+
+#include "core/dom/DOMNodeIds.h"
+#include "core/events/GestureEvent.h"
+#include "core/frame/FrameHost.h"
+#include "core/frame/FrameView.h"
+#include "core/frame/TopControls.h"
+#include "core/html/HTMLFrameOwnerElement.h"
+#include "core/input/EventHandler.h"
+#include "core/layout/LayoutPart.h"
+#include "core/loader/DocumentLoader.h"
+#include "core/page/AutoscrollController.h"
+#include "core/page/Page.h"
+#include "core/page/scrolling/OverscrollController.h"
+#include "core/page/scrolling/RootScroller.h"
+#include "core/page/scrolling/ScrollState.h"
+#include "core/paint/PaintLayer.h"
+#include "platform/PlatformGestureEvent.h"
+
+
+namespace blink {
+
+namespace {
+
+// TODO(bokan): This method can go away once all scrolls happen through the
+// scroll customization path.
+void computeScrollChainForSingleNode(Node& node, std::deque<int>& scrollChain)
+{
+    scrollChain.clear();
+
+    DCHECK(node.layoutObject());
+    Element* element = toElement(&node);
+
+    scrollChain.push_front(DOMNodeIds::idForNode(element));
+}
+
+void recomputeScrollChain(const LocalFrame& frame, const Node& startNode,
+    std::deque<int>& scrollChain)
+{
+    scrollChain.clear();
+
+    DCHECK(startNode.layoutObject());
+    LayoutBox* curBox = startNode.layoutObject()->enclosingBox();
+
+    // Scrolling propagates along the containing block chain.
+    while (curBox && !curBox->isLayoutView()) {
+        Node* curNode = curBox->node();
+        // FIXME: this should reject more elements, as part of crbug.com/410974.
+        if (curNode && curNode->isElementNode()) {
+            Element* curElement = toElement(curNode);
+            if (curElement == frame.document()->scrollingElement())
+                break;
+            scrollChain.push_front(DOMNodeIds::idForNode(curElement));
+        }
+        curBox = curBox->containingBlock();
+    }
+    // TODO(tdresser): this should sometimes be excluded, as part of crbug.com/410974.
+    // We need to ensure that the scrollingElement is always part of
+    // the scroll chain. In quirks mode, when the scrollingElement is
+    // the body, some elements may use the documentElement as their
+    // containingBlock, so we ensure the scrollingElement is added
+    // here.
+    scrollChain.push_front(DOMNodeIds::idForNode(frame.document()->scrollingElement()));
+}
+
+} // namespace
+
+ScrollManager::ScrollManager(LocalFrame* frame)
+: m_frame(frame)
+{
+    clear();
+}
+
+ScrollManager::~ScrollManager()
+{
+}
+
+void ScrollManager::clear()
+{
+    m_lastGestureScrollOverWidget = false;
+    m_scrollbarHandlingScrollGesture = nullptr;
+    m_resizeScrollableArea = nullptr;
+    m_offsetFromResizeCorner = LayoutSize();
+    clearGestureScrollState();
+}
+
+void ScrollManager::clearGestureScrollState()
+{
+    m_scrollGestureHandlingNode = nullptr;
+    m_previousGestureScrolledNode = nullptr;
+    m_deltaConsumedForScrollSequence = false;
+    m_currentScrollChain.clear();
+
+    if (FrameHost* host = frameHost()) {
+        bool resetX = true;
+        bool resetY = true;
+        host->overscrollController().resetAccumulated(resetX, resetY);
+    }
+}
+
+void ScrollManager::stopAutoscroll()
+{
+    if (AutoscrollController* controller = autoscrollController())
+        controller->stopAutoscroll();
+}
+
+bool ScrollManager::panScrollInProgress() const
+{
+    return autoscrollController() && autoscrollController()->panScrollInProgress();
+}
+
+AutoscrollController* ScrollManager::autoscrollController() const
+{
+    if (Page* page = m_frame->page())
+        return &page->autoscrollController();
+    return nullptr;
+}
+
+ScrollResult ScrollManager::physicalScroll(ScrollGranularity granularity,
+    const FloatSize& delta, const FloatPoint& position,
+    const FloatSize& velocity, Node* startNode, Node** stopNode, bool* consumed)
+{
+    if (consumed)
+        *consumed = false;
+    if (delta.isZero())
+        return ScrollResult();
+
+    Node* node = startNode;
+    DCHECK(node && node->layoutObject());
+
+    m_frame->document()->updateStyleAndLayoutIgnorePendingStylesheets();
+
+    ScrollResult result;
+
+    LayoutBox* curBox = node->layoutObject()->enclosingBox();
+    while (curBox) {
+        // If we're at the stopNode, we should try to scroll it but we shouldn't
+        // chain past it.
+        bool shouldStopChaining =
+            stopNode && *stopNode && curBox->node() == *stopNode;
+        bool wasRootScroller = false;
+
+        result = scrollBox(
+            curBox,
+            granularity,
+            delta,
+            position,
+            velocity,
+            &wasRootScroller);
+
+        if (result.didScroll() && stopNode)
+            *stopNode = curBox->node();
+
+        if (result.didScroll() || shouldStopChaining) {
+            setFrameWasScrolledByUser();
+            if (consumed)
+                *consumed = true;
+            return result;
+        }
+        if (wasRootScroller) {
+            // Don't try to chain past the root scroller, even if there's
+            // eligible ancestors.
+            break;
+        }
+
+        curBox = curBox->containingBlock();
+    }
+
+    return result;
+}
+
+bool ScrollManager::logicalScroll(ScrollDirection direction, ScrollGranularity granularity, Node* startNode, Node* mousePressNode)
+{
+    Node* node = startNode;
+
+    if (!node)
+        node = m_frame->document()->focusedElement();
+
+    if (!node)
+        node = mousePressNode;
+
+    if ((!node || !node->layoutObject()) && m_frame->view() && !m_frame->view()->layoutViewItem().isNull())
+        node = m_frame->view()->layoutViewItem().node();
+
+    if (!node)
+        return false;
+
+    m_frame->document()->updateStyleAndLayoutIgnorePendingStylesheets();
+
+    LayoutBox* curBox = node->layoutObject()->enclosingBox();
+    while (curBox) {
+        ScrollDirectionPhysical physicalDirection = toPhysicalDirection(
+            direction, curBox->isHorizontalWritingMode(), curBox->style()->isFlippedBlocksWritingMode());
+
+        ScrollResult result = curBox->scroll(granularity, toScrollDelta(physicalDirection, 1));
+
+        if (result.didScroll()) {
+            setFrameWasScrolledByUser();
+            return true;
+        }
+
+        curBox = curBox->containingBlock();
+    }
+
+    return false;
+}
+
+// TODO(bokan): This should be merged with logicalScroll assuming
+// defaultSpaceEventHandler's chaining scroll can be done crossing frames.
+bool ScrollManager::bubblingScroll(ScrollDirection direction, ScrollGranularity granularity, Node* startingNode, Node* mousePressNode)
+{
+    // The layout needs to be up to date to determine if we can scroll. We may be
+    // here because of an onLoad event, in which case the final layout hasn't been performed yet.
+    m_frame->document()->updateStyleAndLayoutIgnorePendingStylesheets();
+    // FIXME: enable scroll customization in this case. See crbug.com/410974.
+    if (logicalScroll(direction, granularity, startingNode, mousePressNode))
+        return true;
+
+    Frame* parentFrame = m_frame->tree().parent();
+    if (!parentFrame || !parentFrame->isLocalFrame())
+        return false;
+    // FIXME: Broken for OOPI.
+    return toLocalFrame(parentFrame)->eventHandler().bubblingScroll(direction, granularity, m_frame->deprecatedLocalOwner());
+}
+
+ScrollResult ScrollManager::scrollBox(LayoutBox* box,
+    ScrollGranularity granularity, const FloatSize& delta,
+    const FloatPoint& position, const FloatSize& velocity,
+    bool* wasRootScroller)
+{
+    DCHECK(box);
+    Node* node = box->node();
+
+    // If there's no ApplyScroll callback on the element, scroll as usuall in
+    // the non-scroll-customization case.
+    if (!node || !node->isElementNode() || !toElement(node)->getApplyScroll()) {
+        *wasRootScroller = false;
+        return box->scroll(granularity, delta);
+    }
+
+    // Viewport actions should only happen when scrolling an element in the
+    // main frame.
+    DCHECK(m_frame->isMainFrame());
+
+    // If there is an ApplyScroll callback, its because we placed one on the
+    // root scroller to control top controls and overscroll. Invoke a scroll
+    // using parts of the scroll customization framework on just this element.
+    computeScrollChainForSingleNode(*node, m_currentScrollChain);
+
+    OwnPtr<ScrollStateData> scrollStateData = adoptPtr(new ScrollStateData());
+    scrollStateData->delta_x = delta.width();
+    scrollStateData->delta_y = delta.height();
+    scrollStateData->position_x = position.x();
+    scrollStateData->position_y = position.y();
+    // TODO(bokan): delta_granularity is meant to be the number of pixels per
+    // unit of delta but we can't determine that until we get to the area we'll
+    // scroll. This is a hack, we stuff the enum into the double value for
+    // now.
+    scrollStateData->delta_granularity = static_cast<double>(granularity);
+    scrollStateData->velocity_x = velocity.width();
+    scrollStateData->velocity_y = velocity.height();
+    scrollStateData->should_propagate = false;
+    scrollStateData->is_in_inertial_phase = false;
+    scrollStateData->from_user_input = true;
+    scrollStateData->delta_consumed_for_scroll_sequence = false;
+    ScrollState* scrollState =
+        ScrollState::create(std::move(scrollStateData));
+
+    customizedScroll(*node, *scrollState);
+
+    ScrollResult result(
+        scrollState->deltaX() != delta.width(),
+        scrollState->deltaY() != delta.height(),
+        scrollState->deltaX(),
+        scrollState->deltaY());
+
+    *wasRootScroller = true;
+    m_currentScrollChain.clear();
+
+    return result;
+}
+
+void ScrollManager::setFrameWasScrolledByUser()
+{
+    if (DocumentLoader* documentLoader = m_frame->loader().documentLoader())
+        documentLoader->initialScrollState().wasScrolledByUser = true;
+}
+
+void ScrollManager::customizedScroll(const Node& startNode, ScrollState& scrollState)
+{
+    if (scrollState.fullyConsumed())
+        return;
+
+    if (scrollState.deltaX() || scrollState.deltaY())
+        m_frame->document()->updateStyleAndLayoutIgnorePendingStylesheets();
+
+    if (m_currentScrollChain.empty())
+        recomputeScrollChain(*m_frame, startNode, m_currentScrollChain);
+    scrollState.setScrollChain(m_currentScrollChain);
+
+    scrollState.distributeToScrollChainDescendant();
+}
+
+WebInputEventResult ScrollManager::handleGestureScrollBegin(const PlatformGestureEvent& gestureEvent)

[bokan, 2016/05/31 19:39:13]

To me, this and (related methods) seems like it belongs in EventHandler or some
new GestureHandler class which should unpack and interpret the GestureScroll
event. Once it's decided how much and in what way to scroll, it should pass that
information to the ScrollManager to actually execute the scroll.

That said, I'm happy to keep this for now to facilitate breaking up
EventHandler. Once we untangle the mess it should be easier to tell where the
lines of separation should be.

[mustaq, 2016/05/31 20:20:10]

Again, may be a TODO in the class-level comment in .h?

[Navid Zolghadr, 2016/06/01 17:36:43]

I was on the fence on whether to put the scroll gestures in ScrollManager and
touch gestures in TouchManager or put all gestures in one single GestureManager
class. Since you are in favor of putting them all in one as well I'll go with
that then. I have filed an issue and added the TODO in .h file.

+{
+    Document* document = m_frame->document();
+    if (document->layoutViewItem().isNull())
+        return WebInputEventResult::NotHandled;
+
+    FrameView* view = m_frame->view();
+    if (!view)
+        return WebInputEventResult::NotHandled;
+
+    // If there's no layoutObject on the node, send the event to the nearest ancestor with a layoutObject.
+    // Needed for <option> and <optgroup> elements so we can touch scroll <select>s
+    while (m_scrollGestureHandlingNode && !m_scrollGestureHandlingNode->layoutObject())
+        m_scrollGestureHandlingNode = m_scrollGestureHandlingNode->parentOrShadowHostNode();
+
+    if (!m_scrollGestureHandlingNode) {
+        if (RuntimeEnabledFeatures::scrollCustomizationEnabled())
+            m_scrollGestureHandlingNode = m_frame->document()->documentElement();
+        else
+            return WebInputEventResult::NotHandled;
+    }
+    DCHECK(m_scrollGestureHandlingNode);
+
+    passScrollGestureEventToWidget(gestureEvent, m_scrollGestureHandlingNode->layoutObject());
+    if (RuntimeEnabledFeatures::scrollCustomizationEnabled()) {
+        m_currentScrollChain.clear();
+        OwnPtr<ScrollStateData> scrollStateData = adoptPtr(new ScrollStateData());
+        scrollStateData->position_x = gestureEvent.position().x();
+        scrollStateData->position_y = gestureEvent.position().y();
+        scrollStateData->is_beginning = true;
+        scrollStateData->from_user_input = true;
+        scrollStateData->delta_consumed_for_scroll_sequence = m_deltaConsumedForScrollSequence;
+        ScrollState* scrollState = ScrollState::create(std::move(scrollStateData));
+        customizedScroll(*m_scrollGestureHandlingNode.get(), *scrollState);
+    } else {
+        if (m_frame->isMainFrame())
+            m_frame->host()->topControls().scrollBegin();
+    }
+    return WebInputEventResult::HandledSystem;
+}
+
+WebInputEventResult ScrollManager::handleGestureScrollUpdate(const PlatformGestureEvent& gestureEvent)
+{
+    DCHECK_EQ(gestureEvent.type(), PlatformEvent::GestureScrollUpdate);
+
+    // Negate the deltas since the gesture event stores finger movement and
+    // scrolling occurs in the direction opposite the finger's movement
+    // direction. e.g. Finger moving up has negative event delta but causes the
+    // page to scroll down causing positive scroll delta.
+    FloatSize delta(-gestureEvent.deltaX(), -gestureEvent.deltaY());
+    FloatSize velocity(-gestureEvent.velocityX(), -gestureEvent.velocityY());
+    if (delta.isZero())
+        return WebInputEventResult::NotHandled;
+
+    ScrollGranularity granularity = gestureEvent.deltaUnits();
+    Node* node = m_scrollGestureHandlingNode.get();
+
+    // Scroll customization is only available for touch.
+    bool handleScrollCustomization = RuntimeEnabledFeatures::scrollCustomizationEnabled() && gestureEvent.source() == PlatformGestureSourceTouchscreen;
+    if (node) {
+        LayoutObject* layoutObject = node->layoutObject();
+        if (!layoutObject)
+            return WebInputEventResult::NotHandled;
+
+        // Try to send the event to the correct view.
+        WebInputEventResult result = passScrollGestureEventToWidget(gestureEvent, layoutObject);
+        if (result != WebInputEventResult::NotHandled) {
+            if (gestureEvent.preventPropagation()
+                && !RuntimeEnabledFeatures::scrollCustomizationEnabled()) {
+                // This is an optimization which doesn't apply with
+                // scroll customization enabled.
+                m_previousGestureScrolledNode = m_scrollGestureHandlingNode;
+            }
+            // FIXME: we should allow simultaneous scrolling of nested
+            // iframes along perpendicular axes. See crbug.com/466991.
+            m_deltaConsumedForScrollSequence = true;
+            return result;
+        }
+
+        if (handleScrollCustomization) {
+            OwnPtr<ScrollStateData> scrollStateData = adoptPtr(new ScrollStateData());
+            scrollStateData->delta_x = delta.width();
+            scrollStateData->delta_y = delta.height();
+            scrollStateData->delta_granularity = ScrollByPrecisePixel;
+            scrollStateData->velocity_x = velocity.width();
+            scrollStateData->velocity_y = velocity.height();
+            scrollStateData->should_propagate = !gestureEvent.preventPropagation();
+            scrollStateData->is_in_inertial_phase = gestureEvent.inertialPhase() == ScrollInertialPhaseMomentum;
+            scrollStateData->from_user_input = true;
+            scrollStateData->delta_consumed_for_scroll_sequence = m_deltaConsumedForScrollSequence;
+            ScrollState* scrollState = ScrollState::create(std::move(scrollStateData));
+            if (m_previousGestureScrolledNode) {
+                // The ScrollState needs to know what the current
+                // native scrolling element is, so that for an
+                // inertial scroll that shouldn't propagate, only the
+                // currently scrolling element responds.
+                DCHECK(m_previousGestureScrolledNode->isElementNode());
+                scrollState->setCurrentNativeScrollingElement(toElement(m_previousGestureScrolledNode.get()));
+            }
+            customizedScroll(*node, *scrollState);
+            m_previousGestureScrolledNode = scrollState->currentNativeScrollingElement();
+            m_deltaConsumedForScrollSequence = scrollState->deltaConsumedForScrollSequence();
+            if (scrollState->deltaX() != delta.width()
+                || scrollState->deltaY() != delta.height()) {
+                setFrameWasScrolledByUser();
+                return WebInputEventResult::HandledSystem;
+            }
+        } else {
+            Node* stopNode = nullptr;
+            if (gestureEvent.preventPropagation())
+                stopNode = m_previousGestureScrolledNode.get();
+
+            bool consumed = false;
+            ScrollResult result = physicalScroll(
+                granularity,
+                delta,
+                FloatPoint(gestureEvent.position()),
+                velocity,
+                node,
+                &stopNode,
+                &consumed);
+
+            if (gestureEvent.preventPropagation())
+                m_previousGestureScrolledNode = stopNode;
+
+            if ((!stopNode || !isRootScroller(*stopNode)) && frameHost()) {
+                frameHost()->overscrollController().resetAccumulated(
+                    result.didScrollX, result.didScrollY);
+            }
+
+            if (consumed)
+                return WebInputEventResult::HandledSystem;
+        }
+    }
+
+    return WebInputEventResult::NotHandled;
+}
+
+WebInputEventResult ScrollManager::handleGestureScrollEnd(const PlatformGestureEvent& gestureEvent)
+{
+    Node* node = m_scrollGestureHandlingNode;
+
+    if (node) {
+        passScrollGestureEventToWidget(gestureEvent, node->layoutObject());
+        if (RuntimeEnabledFeatures::scrollCustomizationEnabled()) {
+            OwnPtr<ScrollStateData> scrollStateData = adoptPtr(new ScrollStateData());
+            scrollStateData->is_ending = true;
+            scrollStateData->is_in_inertial_phase = gestureEvent.inertialPhase() == ScrollInertialPhaseMomentum;
+            scrollStateData->from_user_input = true;
+            scrollStateData->is_direct_manipulation = true;
+            scrollStateData->delta_consumed_for_scroll_sequence = m_deltaConsumedForScrollSequence;
+            ScrollState* scrollState = ScrollState::create(std::move(scrollStateData));
+            customizedScroll(*node, *scrollState);
+        }
+    }
+
+    clearGestureScrollState();
+    return WebInputEventResult::NotHandled;
+}
+
+FrameHost* ScrollManager::frameHost() const
+{
+    if (!m_frame->page())
+        return nullptr;
+
+    return &m_frame->page()->frameHost();
+}
+
+WebInputEventResult ScrollManager::passScrollGestureEventToWidget(const PlatformGestureEvent& gestureEvent, LayoutObject* layoutObject)
+{
+    DCHECK(gestureEvent.isScrollEvent());
+
+    if (!m_lastGestureScrollOverWidget || !layoutObject || !layoutObject->isLayoutPart())
+        return WebInputEventResult::NotHandled;
+
+    Widget* widget = toLayoutPart(layoutObject)->widget();
+
+    if (!widget || !widget->isFrameView())
+        return WebInputEventResult::NotHandled;
+
+    return toFrameView(widget)->frame().eventHandler().handleGestureScrollEvent(gestureEvent);
+}
+
+bool ScrollManager::isRootScroller(const Node& node) const
+{
+    // The root scroller is the one Element on the page designated to perform
+    // "viewport actions" like top controls movement and overscroll glow.
+    if (!frameHost() || !frameHost()->rootScroller())
+        return false;
+
+    return frameHost()->rootScroller()->get() == &node;
+}
+
+
+WebInputEventResult ScrollManager::handleGestureScrollEvent(const PlatformGestureEvent& gestureEvent)
+{
+    Node* eventTarget = nullptr;
+    Scrollbar* scrollbar = nullptr;
+    if (gestureEvent.type() != PlatformEvent::GestureScrollBegin) {
+        scrollbar = m_scrollbarHandlingScrollGesture.get();
+        eventTarget = m_scrollGestureHandlingNode.get();
+    }
+
+    if (!eventTarget) {
+        Document* document = m_frame->document();
+        if (document->layoutViewItem().isNull())
+            return WebInputEventResult::NotHandled;
+
+        FrameView* view = m_frame->view();
+        LayoutPoint viewPoint = view->rootFrameToContents(gestureEvent.position());
+        HitTestRequest request(HitTestRequest::ReadOnly);
+        HitTestResult result(request, viewPoint);
+        document->layoutViewItem().hitTest(result);
+
+        eventTarget = result.innerNode();
+
+        m_lastGestureScrollOverWidget = result.isOverWidget();
+        m_scrollGestureHandlingNode = eventTarget;
+        m_previousGestureScrolledNode = nullptr;
+
+        if (!scrollbar)
+            scrollbar = result.scrollbar();
+    }
+
+    if (scrollbar) {
+        bool shouldUpdateCapture = false;
+        if (scrollbar->gestureEvent(gestureEvent, &shouldUpdateCapture)) {
+            if (shouldUpdateCapture)
+                m_scrollbarHandlingScrollGesture = scrollbar;
+            return WebInputEventResult::HandledSuppressed;
+        }
+        m_scrollbarHandlingScrollGesture = nullptr;
+    }
+
+    if (eventTarget) {
+        if (handleScrollGestureOnResizer(eventTarget, gestureEvent))
+            return WebInputEventResult::HandledSuppressed;
+
+        GestureEvent* gestureDomEvent = GestureEvent::create(eventTarget->document().domWindow(), gestureEvent);
+        if (gestureDomEvent) {
+            DispatchEventResult gestureDomEventResult = eventTarget->dispatchEvent(gestureDomEvent);
+            if (gestureDomEventResult != DispatchEventResult::NotCanceled) {
+                DCHECK(gestureDomEventResult != DispatchEventResult::CanceledByEventHandler);
+                return EventHandler::toWebInputEventResult(gestureDomEventResult);
+            }
+        }
+    }
+
+    switch (gestureEvent.type()) {
+    case PlatformEvent::GestureScrollBegin:
+        return handleGestureScrollBegin(gestureEvent);
+    case PlatformEvent::GestureScrollUpdate:
+        return handleGestureScrollUpdate(gestureEvent);
+    case PlatformEvent::GestureScrollEnd:
+        return handleGestureScrollEnd(gestureEvent);
+    case PlatformEvent::GestureFlingStart:
+    case PlatformEvent::GesturePinchBegin:
+    case PlatformEvent::GesturePinchEnd:
+    case PlatformEvent::GesturePinchUpdate:
+        return WebInputEventResult::NotHandled;
+    default:
+        NOTREACHED();
+        return WebInputEventResult::NotHandled;
+    }
+}
+
+bool ScrollManager::isScrollbarHandlingGestures() const
+{
+    return m_scrollbarHandlingScrollGesture.get();
+}
+
+bool ScrollManager::handleScrollGestureOnResizer(Node* eventTarget, const PlatformGestureEvent& gestureEvent)
+{
+    if (gestureEvent.type() == PlatformEvent::GestureScrollBegin) {
+        PaintLayer* layer = eventTarget->layoutObject() ? eventTarget->layoutObject()->enclosingLayer() : nullptr;
+        IntPoint p = m_frame->view()->rootFrameToContents(gestureEvent.position());
+        if (layer && layer->getScrollableArea() && layer->getScrollableArea()->isPointInResizeControl(p, ResizerForTouch)) {
+            m_resizeScrollableArea = layer->getScrollableArea();
+            m_resizeScrollableArea->setInResizeMode(true);
+            m_offsetFromResizeCorner = LayoutSize(m_resizeScrollableArea->offsetFromResizeCorner(p));
+            return true;
+        }
+    } else if (gestureEvent.type() == PlatformEvent::GestureScrollUpdate) {
+        if (m_resizeScrollableArea && m_resizeScrollableArea->inResizeMode()) {
+            m_resizeScrollableArea->resize(gestureEvent, m_offsetFromResizeCorner);
+            return true;
+        }
+    } else if (gestureEvent.type() == PlatformEvent::GestureScrollEnd) {
+        if (m_resizeScrollableArea && m_resizeScrollableArea->inResizeMode()) {
+            m_resizeScrollableArea->setInResizeMode(false);
+            m_resizeScrollableArea = nullptr;
+            return false;
+        }
+    }
+
+    return false;
+}
+
+bool ScrollManager::inResizeMode() const
+{
+    return m_resizeScrollableArea && m_resizeScrollableArea->inResizeMode();
+}
+
+void ScrollManager::resize(const PlatformEvent& evt)
+{
+    m_resizeScrollableArea->resize(evt, m_offsetFromResizeCorner);
+}
+
+void ScrollManager::clearResizeScrollableArea(bool shouldNotBeNull)
+{
+    if (shouldNotBeNull)
+        DCHECK(m_resizeScrollableArea);
+
+    if (m_resizeScrollableArea)
+        m_resizeScrollableArea->setInResizeMode(false);
+    m_resizeScrollableArea = nullptr;
+}
+
+void ScrollManager::setResizeScrollableArea(PaintLayer* layer, IntPoint p)
+{
+    m_resizeScrollableArea = layer->getScrollableArea();
+    m_resizeScrollableArea->setInResizeMode(true);
+    m_offsetFromResizeCorner = LayoutSize(m_resizeScrollableArea->offsetFromResizeCorner(p));
+}
+
+bool ScrollManager::canHandleGestureEvent(const GestureEventWithHitTestResults& targetedEvent)
+{
+    Scrollbar* scrollbar = targetedEvent.hitTestResult().scrollbar();
+
+    if (scrollbar) {
+        bool shouldUpdateCapture = false;
+        if (scrollbar->gestureEvent(targetedEvent.event(), &shouldUpdateCapture)) {
+            if (shouldUpdateCapture)
+                m_scrollbarHandlingScrollGesture = scrollbar;
+            return true;
+        }
+    }
+    return false;
+}
+
+DEFINE_TRACE(ScrollManager)
+{
+    visitor->trace(m_frame);
+    visitor->trace(m_scrollGestureHandlingNode);
+    visitor->trace(m_previousGestureScrolledNode);
+    visitor->trace(m_scrollbarHandlingScrollGesture);
+    visitor->trace(m_resizeScrollableArea);
+}
+
+} // namespace blink