Implement overlay scrollbar fade out for non-composited scrollers.

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

 /*
  * Copyright (C) 2004, 2006, 2008 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 444 matching lines @@
     m_scrollableArea->mouseEnteredScrollbar(*this);
 }
 
 void Scrollbar::mouseExited() {
   if (m_scrollableArea)
     m_scrollableArea->mouseExitedScrollbar(*this);
   setHoveredPart(NoPart);
 }
 
 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.
     ScrollbarPart part = theme().hitTest(*this, mouseEvent.position());
     if (part == NoPart)
       m_scrollableArea->mouseExitedScrollbar(*this);
   }
 }
 
 void Scrollbar::mouseDown(const PlatformMouseEvent& evt) {
@@ skipping 14 matching lines @@
     int thumbLen = theme().thumbLength(*this);
     int desiredPos = pressedPos;
     // Set the pressed position to the middle of the thumb so that when we do
     // the move, the delta will be from the current pixel position of the thumb
     // to the new desired position for the thumb.
     m_pressedPos = theme().trackPosition(*this) + theme().thumbPosition(*this) +
                    thumbLen / 2;
     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) {
   if (m_enabled == e)
     return;
   m_enabled = e;
   theme().updateEnabledState(*this);
   setNeedsPaintInvalidation(AllParts);
 }
 
 int Scrollbar::scrollbarThickness() const {
   int thickness = orientation() == HorizontalScrollbar ? height() : width();
   if (!thickness || !m_hostWindow)
     return thickness;
   return m_hostWindow->windowToViewportScalar(m_themeScrollbarThickness);
 }
 
 bool Scrollbar::isOverlayScrollbar() const {
   return m_theme.usesOverlayScrollbars();
 }
 
 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 {
   return m_scrollableArea && m_scrollableArea->isActive();
 }
 
 IntRect Scrollbar::convertToContainingWidget(const IntRect& localRect) const {
   if (m_scrollableArea)
     return m_scrollableArea->convertFromScrollbarToContainingWidget(*this,
                                                                     localRect);
@@ skipping 64 matching lines @@
     invalidParts = AllParts;
   if (invalidParts & ~ThumbPart)
     m_trackNeedsRepaint = true;
   if (invalidParts & ThumbPart)
     m_thumbNeedsRepaint = true;
   if (m_scrollableArea)
     m_scrollableArea->setScrollbarNeedsPaintInvalidation(orientation());
 }
 
 }  // namespace blink