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

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

 /*
  * Copyright (C) 2011 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 189 matching lines @@
     return Platform::current()->scrollbarBehavior()->shouldCenterOnThumb(static_cast<WebScrollbarBehavior::Button>(evt.button()), evt.shiftKey(), evt.altKey());
 }
 
 bool ScrollbarTheme::shouldSnapBackToDragOrigin(const ScrollbarThemeClient& scrollbar, const PlatformMouseEvent& evt)
 {
     IntPoint mousePosition = scrollbar.convertFromRootFrame(evt.position());
     mousePosition.move(scrollbar.x(), scrollbar.y());
     return Platform::current()->scrollbarBehavior()->shouldSnapBackToDragOrigin(mousePosition, trackRect(scrollbar), scrollbar.orientation() == HorizontalScrollbar);
 }
 
-int ScrollbarTheme::thumbPosition(const ScrollbarThemeClient& scrollbar)
+int ScrollbarTheme::thumbPosition(const ScrollbarThemeClient& scrollbar, float scrollPosition)
 {
     if (scrollbar.enabled()) {
         float size = scrollbar.totalSize() - scrollbar.visibleSize();
         // Avoid doing a floating point divide by zero and return 1 when usedTotalSize == visibleSize.
         if (!size)
             return 0;
-        float pos = std::max(0.0f, scrollbar.currentPos()) * (trackLength(scrollbar) - thumbLength(scrollbar)) / size;
+        float pos = std::max(0.0f, scrollPosition) * (trackLength(scrollbar) - thumbLength(scrollbar)) / size;
         return (pos < 1 && pos > 0) ? 1 : pos;
     }
     return 0;
 }
 
 int ScrollbarTheme::thumbLength(const ScrollbarThemeClient& scrollbar)
 {
     if (!scrollbar.enabled())
         return 0;
 
@@ skipping 113 matching lines @@
         return DisplayItem::ScrollbarBackTrack;
     case ForwardTrackPart:
         return DisplayItem::ScrollbarForwardTrack;
     default:
         ASSERT_NOT_REACHED();
         return DisplayItem::ScrollbarBackTrack;
     }
 }
 
 } // namespace blink