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

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

 /*
  * Copyright (C) 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 12 matching lines @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #ifndef ScrollbarTheme_h
 #define ScrollbarTheme_h
 
 #include "platform/PlatformExport.h"
 #include "platform/geometry/IntRect.h"
 #include "platform/graphics/paint/DisplayItem.h"
 #include "platform/scroll/ScrollTypes.h"
+#include "platform/scroll/ScrollbarThemeClient.h"
 #include "public/platform/WebScrollbarButtonsPlacement.h"
 
 namespace blink {
 
 class CullRect;
 class GraphicsContext;
 class PlatformMouseEvent;
-class ScrollbarThemeClient;
 class ScrollbarThemePaintParams;
 
 class PLATFORM_EXPORT ScrollbarTheme {
     WTF_MAKE_NONCOPYABLE(ScrollbarTheme); USING_FAST_MALLOC(ScrollbarTheme);
 public:
     ScrollbarTheme() { }
     virtual ~ScrollbarTheme() { }
 
     // If true, then scrollbars with this theme will be painted every time
     // Scrollbar::setNeedsPaintInvalidation is called. If false, then only parts
@@ skipping 27 matching lines @@
     }
 
     virtual void paintScrollCorner(GraphicsContext&, const DisplayItemClient&, const IntRect& cornerRect);
     virtual void paintTickmarks(GraphicsContext&, const ScrollbarThemeClient&, const IntRect&) { }
 
     virtual bool shouldCenterOnThumb(const ScrollbarThemeClient&, const PlatformMouseEvent&);
     virtual bool shouldSnapBackToDragOrigin(const ScrollbarThemeClient&, const PlatformMouseEvent&);
     virtual bool shouldDragDocumentInsteadOfThumb(const ScrollbarThemeClient&, const PlatformMouseEvent&) { return false; }
 
     // The position of the thumb relative to the track.
-    virtual int thumbPosition(const ScrollbarThemeClient&);
+    int thumbPosition(const ScrollbarThemeClient& scrollbar) { return thumbPosition(scrollbar, scrollbar.currentPos()); }
+    // The position the thumb would have, relative to the track, at the specified scroll position.
+    virtual int thumbPosition(const ScrollbarThemeClient&, float scrollPosition);
     // The length of the thumb along the axis of the scrollbar.
     virtual int thumbLength(const ScrollbarThemeClient&);
     // The position of the track relative to the scrollbar.
     virtual int trackPosition(const ScrollbarThemeClient&);
     // The length of the track along the axis of the scrollbar.
     virtual int trackLength(const ScrollbarThemeClient&);
     // The opacity to be applied to the thumb.
     virtual float thumbOpacity(const ScrollbarThemeClient&) const { return 1.0f; }
 
     virtual bool hasButtons(const ScrollbarThemeClient&) = 0;
@@ skipping 36 matching lines @@
     static DisplayItem::Type buttonPartToDisplayItemType(ScrollbarPart);
     static DisplayItem::Type trackPiecePartToDisplayItemType(ScrollbarPart);
 
 private:
     static ScrollbarTheme& nativeTheme(); // Must be implemented to return the correct theme subclass.
     static bool gMockScrollbarsEnabled;
 };
 
 }
 #endif