Implement overlay scrollbar fade out for non-composited scrollers.

third_party/WebKit/Source/platform/scroll/Scrollbar.cpp

Index: third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp b/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
index 71273852b4c469c5fb9cd44d7b4113cf02a48fda..516eed533a8be643ee4b9a1c0b32f7df95180df5 100644
--- a/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
+++ b/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
@@ -462,12 +462,16 @@ void Scrollbar::mouseExited() {
 }
 
 void Scrollbar::mouseUp(const PlatformMouseEvent& mouseEvent) {
+  bool isCaptured = m_pressedPart == ThumbPart;

[skobes, 2016/11/02 21:02:43]

Does this work if the mouse is pressed on the thumb but then moves off of the
thumb before being released?

[bokan, 2016/11/02 22:41:57]

Yes, there's a bunch of logic in EventHandler for this though it's not great and
I found some edge cases but we'll fix those separately. The basic usage works as
you'd expect.

   setPressedPart(NoPart);
   m_pressedPos = 0;
   m_draggingDocument = false;
   stopTimerIfNeeded();
 
   if (m_scrollableArea) {
+    if (isCaptured)
+      m_scrollableArea->mouseReleasedScrollbar();
+
     // m_hoveredPart won't be updated until the next mouseMoved or mouseDown, so
     // we have to hit test to really know if the mouse has exited the scrollbar
     // on a mouseUp.
@@ -502,17 +506,20 @@ void Scrollbar::mouseDown(const PlatformMouseEvent& evt) {
     moveThumb(desiredPos);
     return;
   }
-  if (m_pressedPart == ThumbPart)
+  if (m_pressedPart == ThumbPart) {
     m_dragOrigin = m_currentPos;
+    if (m_scrollableArea)
+      m_scrollableArea->mouseCapturedScrollbar();
+  }
 
   m_pressedPos = pressedPos;
 
   autoscrollPressedPart(theme().initialAutoscrollTimerDelay());
 }
 
-void Scrollbar::visibilityChanged() {
+void Scrollbar::setScrollbarsHidden(bool hidden) {
   if (m_scrollableArea)
-    m_scrollableArea->scrollbarVisibilityChanged();
+    m_scrollableArea->setScrollbarsHidden(hidden);
 }
 
 void Scrollbar::setEnabled(bool e) {
@@ -538,8 +545,7 @@ bool Scrollbar::shouldParticipateInHitTesting() {
   // Non-overlay scrollbars should always participate in hit testing.
   if (!isOverlayScrollbar())
     return true;
-  return m_scrollableArea->scrollAnimator()
-      .shouldScrollbarParticipateInHitTesting(*this);
+  return !m_scrollableArea->scrollbarsHidden();
 }
 
 bool Scrollbar::isWindowActive() const {