Fix smooth scroll overshooting when mouse held down in scrollbar track.

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 163 matching lines @@
 
     if (!theme().paint(*this, context, cullRect))
         Widget::paint(context, cullRect);
 }
 
 void Scrollbar::autoscrollTimerFired(Timer<Scrollbar>*)
 {
     autoscrollPressedPart(theme().autoscrollTimerDelay());
 }
 
-static bool thumbUnderMouse(Scrollbar& scrollbar)
+bool Scrollbar::thumbWillBeUnderMouse() const
 {
-    int thumbPos = scrollbar.theme().trackPosition(scrollbar) + scrollbar.theme().thumbPosition(scrollbar);
-    int thumbLength = scrollbar.theme().thumbLength(scrollbar);
-    return scrollbar.pressedPos() >= thumbPos && scrollbar.pressedPos() < thumbPos + thumbLength;
+    int thumbPos = theme().trackPosition(*this) + theme().thumbPosition(*this, scrollableAreaTargetPos());
+    int thumbLength = theme().thumbLength(*this);
+    return pressedPos() >= thumbPos && pressedPos() < thumbPos + thumbLength;
 }
 
 void Scrollbar::autoscrollPressedPart(double delay)
 {
     // Don't do anything for the thumb or if nothing was pressed.
     if (m_pressedPart == ThumbPart || m_pressedPart == NoPart)
         return;
 
     // Handle the track.
-    if ((m_pressedPart == BackTrackPart || m_pressedPart == ForwardTrackPart) && thumbUnderMouse(*this)) {
+    if ((m_pressedPart == BackTrackPart || m_pressedPart == ForwardTrackPart) && thumbWillBeUnderMouse()) {
         setHoveredPart(ThumbPart);
         return;
     }
 
     // Handle the arrows and track.
     if (m_scrollableArea && m_scrollableArea->userScroll(pressedPartScrollDirectionPhysical(), pressedPartScrollGranularity()).didScroll)
         startTimerIfNeeded(delay);
 }
 
 void Scrollbar::startTimerIfNeeded(double delay)
 {
     // Don't do anything for the thumb.
     if (m_pressedPart == ThumbPart)
         return;
 
     // Handle the track.  We halt track scrolling once the thumb is level
     // with us.
-    if ((m_pressedPart == BackTrackPart || m_pressedPart == ForwardTrackPart) && thumbUnderMouse(*this)) {
+    if ((m_pressedPart == BackTrackPart || m_pressedPart == ForwardTrackPart) && thumbWillBeUnderMouse()) {
         setHoveredPart(ThumbPart);
         return;
     }
 
     // We can't scroll if we've hit the beginning or end.
     ScrollDirectionPhysical dir = pressedPartScrollDirectionPhysical();
     if (dir == ScrollUp || dir == ScrollLeft) {
         if (m_currentPos == 0)
             return;
     } else {
@@ skipping 328 matching lines @@
 {
     if (!m_scrollableArea)
         return 0;
 
     if (m_orientation == HorizontalScrollbar)
         return m_scrollableArea->scrollPosition().x() - m_scrollableArea->minimumScrollPosition().x();
 
     return m_scrollableArea->scrollPosition().y() - m_scrollableArea->minimumScrollPosition().y();
 }
 
+float Scrollbar::scrollableAreaTargetPos() const
+{
+    if (!m_scrollableArea)
+        return 0;
+
+    if (m_orientation == HorizontalScrollbar)
+        return m_scrollableArea->scrollAnimator().desiredTargetPosition().x() - m_scrollableArea->minimumScrollPosition().x();
+
+    return m_scrollableArea->scrollAnimator().desiredTargetPosition().y() - m_scrollableArea->minimumScrollPosition().y();
+}
+
 void Scrollbar::setNeedsPaintInvalidation(ScrollbarPart invalidParts)
 {
     if (m_theme.shouldRepaintAllPartsOnInvalidation())
         invalidParts = AllParts;
     if (invalidParts & ~ThumbPart)
         m_trackNeedsRepaint = true;
     if (invalidParts & ThumbPart)
         m_thumbNeedsRepaint = true;
     if (m_scrollableArea)
         m_scrollableArea->setScrollbarNeedsPaintInvalidation(orientation());
 }
 
 } // namespace blink