Add ScrollManager class

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

+// 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/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;
+}
+
+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)
+{
+    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();
+    const PlatformGestureEvent& gestureEvent = targetedEvent.event();

[dtapuska, 2016/05/27 14:12:23]

do you need to put this on the stack? Should be able to just pass it in.

[Navid Zolghadr, 2016/05/30 20:15:47]

Yup. You are right. 

+
+    if (scrollbar) {
+        bool shouldUpdateCapture = false;
+        if (scrollbar->gestureEvent(gestureEvent, &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