Handle gesture scroll events on scrollbars from Touchpad.

With wheel gesture event generation wheel events inside scrollbars
were not working correctly. The scrollbar was only entering the
capturing phase on gesture tap down which doesn't occur with Touchpad.
Transition capturing state on GestureScrollBegin when over a scrollbar.

LayoutTest/fast/events/wheel-in-scrollbar.html LayoutTest covers testing
this when wheel event gestures are enabled.

BUG=568183

Review URL: https://codereview.chromium.org/1805063003

Cr-Commit-Position: refs/heads/master@{#381988}

1 files changed, 41 insertions, 6 deletions

diff --git a/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp b/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
index f1fa7bc..b340a59 100644
--- a/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
+++ b/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
@@ -325,25 +325,60 @@ void Scrollbar::setPressedPart(ScrollbarPart part)
     m_pressedPart = part;
 }
 
-bool Scrollbar::gestureEvent(const PlatformGestureEvent& evt)
+bool Scrollbar::gestureEvent(const PlatformGestureEvent& evt, bool* shouldUpdateCapture)
 {
+    DCHECK(shouldUpdateCapture);
     switch (evt.type()) {
     case PlatformEvent::GestureTapDown:
         setPressedPart(theme().hitTest(*this, evt.position()));
         m_pressedPos = orientation() == HorizontalScrollbar ? convertFromRootFrame(evt.position()).x() : convertFromRootFrame(evt.position()).y();
+        *shouldUpdateCapture = true;
         return true;
     case PlatformEvent::GestureTapDownCancel:
-    case PlatformEvent::GestureScrollBegin:
         if (m_pressedPart != ThumbPart)
             return false;
         m_scrollPos = m_pressedPos;
         return true;
+    case PlatformEvent::GestureScrollBegin:
+        switch (evt.source()) {
+        case PlatformGestureSourceTouchpad:
+            // Update the state on GSB for touchpad since GestureTapDown
+            // is not generated by that device. Touchscreen uses the tap down
+            // gesture since the scrollbar enters a visual active state.
+            *shouldUpdateCapture = true;
+            setPressedPart(NoPart);
+            m_pressedPos = 0;
+            return true;
+        case PlatformGestureSourceTouchscreen:
+            if (m_pressedPart != ThumbPart)
+                return false;
+            m_scrollPos = m_pressedPos;
+            return true;
+        default:
+            ASSERT_NOT_REACHED();
+            return true;
+        }
+        break;
     case PlatformEvent::GestureScrollUpdate:
-        if (m_pressedPart != ThumbPart)
+        switch (evt.source()) {
+        case PlatformGestureSourceTouchpad: {
+            FloatSize delta(-evt.deltaX(), -evt.deltaY());
+            if (m_scrollableArea && m_scrollableArea->userScroll(evt.deltaUnits(), delta).didScroll()) {
+                return true;
+            }
             return false;
-        m_scrollPos += orientation() == HorizontalScrollbar ? evt.deltaX() : evt.deltaY();
-        moveThumb(m_scrollPos, false);
-        return true;
+        }
+        case PlatformGestureSourceTouchscreen:
+            if (m_pressedPart != ThumbPart)
+                return false;
+            m_scrollPos += orientation() == HorizontalScrollbar ? evt.deltaX() : evt.deltaY();
+            moveThumb(m_scrollPos, false);
+            return true;
+        default:
+            ASSERT_NOT_REACHED();
+            return true;
+        }
+        break;
     case PlatformEvent::GestureScrollEnd:
     case PlatformEvent::GestureLongPress:
     case PlatformEvent::GestureFlingStart: