Make ScrollbarThemeAura selectively invalidate scrollbar parts.

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 29 matching lines @@
 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 scrollbar
-    // thumb and track part painting results will be cached and not repainted
-    // unless requested by Scrollbar::setThumbNeedsRepaint or
-    // Scrollbar::setTrackNeedsRepaint.
+    // Scrollbar::setNeedsPaintInvalidation is called. If false, then only parts
+    // which are explicitly invalidated will be repainted.
     virtual bool shouldRepaintAllPartsOnInvalidation() const { return true; }
 
     virtual void updateEnabledState(const ScrollbarThemeClient&) { }
 
     virtual bool paint(const ScrollbarThemeClient&, GraphicsContext&, const CullRect&);
 
     virtual ScrollbarPart hitTest(const ScrollbarThemeClient&, const IntPoint&);
 
     virtual int scrollbarThickness(ScrollbarControlSize = RegularScrollbar) { return 0; }
     virtual int scrollbarMargin() const { return 0; }
 
     virtual WebScrollbarButtonsPlacement buttonsPlacement() const { return WebScrollbarButtonsPlacementSingle; }
 
     virtual bool supportsControlTints() const { return false; }
     virtual bool usesOverlayScrollbars() const { return false; }
     virtual void updateScrollbarOverlayStyle(const ScrollbarThemeClient&) { }
 
     virtual bool invalidateOnMouseEnterExit() { return false; }
     virtual bool invalidateOnWindowActiveChange() const { return false; }
 
+    // Returns parts of the scrollbar which must be repainted following a change
+    // in the thumb position, given scroll positions before and after.
+    virtual ScrollbarPart invalidateOnThumbPositionChange(
+        const ScrollbarThemeClient&, float oldPosition, float newPosition) const
+    {
+        return AllParts;
+    }
+
     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&);
     // The length of the thumb along the axis of the scrollbar.
@@ skipping 45 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