Reland: Make ScrollbarThemeAura selectively invalidate scrollbar parts.

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

 /*
  * Copyright (C) 2004, 2006 Apple Computer, 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 76 matching lines @@
 
     ScrollbarPart pressedPart() const override { return m_pressedPart; }
     ScrollbarPart hoveredPart() const override { return m_hoveredPart; }
 
     void styleChanged() override { }
     void visibilityChanged() override;
     bool enabled() const override { return m_enabled; }
     void setEnabled(bool) override;
 
     // Called by the ScrollableArea when the scroll offset changes.
-    void offsetDidChange();
+    //
+    // Will invalidate the scrollbar if either the track or the thumb is
+    // invalidated. The caller is responsible for issuing paint invalidations
+    // when only the thumb position changes, as the scrollbar is unaware of
+    // whether the thumb can be moved without repainting.
+    //
+    // Returns true if the scrollbar's offset was actually updated, so that the
+    // caller can issue additional invalidations as needed.
+    bool offsetDidChange();
 
     void disconnectFromScrollableArea();
     ScrollableArea* scrollableArea() const { return m_scrollableArea; }
 
     int pressedPos() const { return m_pressedPos; }
 
     virtual void setHoveredPart(ScrollbarPart);
     virtual void setPressedPart(ScrollbarPart);
 
     void setProportion(int visibleSize, int totalSize);
@@ skipping 26 matching lines @@
     IntRect convertFromContainingWidget(const IntRect&) const override;
 
     IntPoint convertToContainingWidget(const IntPoint&) const override;
     IntPoint convertFromContainingWidget(const IntPoint&) const override;
 
     void moveThumb(int pos, bool draggingDocument = false);
 
     float elasticOverscroll() const override { return m_elasticOverscroll; }
     void setElasticOverscroll(float elasticOverscroll) override { m_elasticOverscroll = elasticOverscroll; }
 
+    // Use setNeedsPaintInvalidation to cause scrollbar parts to repaint.
     bool trackNeedsRepaint() const { return m_trackNeedsRepaint; }
     void setTrackNeedsRepaint(bool trackNeedsRepaint) { m_trackNeedsRepaint = trackNeedsRepaint; }
     bool thumbNeedsRepaint() const { return m_thumbNeedsRepaint; }
     void setThumbNeedsRepaint(bool thumbNeedsRepaint) { m_thumbNeedsRepaint = thumbNeedsRepaint; }
 
     bool overlapsResizer() const { return m_overlapsResizer; }
     void setOverlapsResizer(bool overlapsResizer) { m_overlapsResizer = overlapsResizer; }
 
     // DisplayItemClient methods.
     String debugName() const final { return m_orientation == HorizontalScrollbar ? "HorizontalScrollbar" : "VerticalScrollbar"; }
     // TODO(chrishtr): fix this.
     IntRect visualRect() const override { return IntRect(); }
 
-    void setNeedsPaintInvalidation();
+    // Marks the specified parts of the scrollbar as needing paint invalidation.
+    // Uses the associated ScrollableArea to cause invalidation.
+    void setNeedsPaintInvalidation(ScrollbarPart = AllParts);
 
     // Promptly unregister from the theme manager + run finalizers of derived Scrollbars.
     EAGERLY_FINALIZE();
 #if ENABLE(OILPAN)
     DECLARE_EAGER_FINALIZATION_OPERATOR_NEW();
 #endif
     DECLARE_VIRTUAL_TRACE();
 
 protected:
     Scrollbar(ScrollableArea*, ScrollbarOrientation, ScrollbarControlSize, ScrollbarTheme* = 0);
 
-    void updateThumb();
 
     void autoscrollTimerFired(Timer<Scrollbar>*);
     void startTimerIfNeeded(double delay);
     void stopTimerIfNeeded();
     void autoscrollPressedPart(double delay);
     ScrollDirectionPhysical pressedPartScrollDirectionPhysical();
     ScrollGranularity pressedPartScrollGranularity();
 
     RawPtrWillBeMember<ScrollableArea> m_scrollableArea;
     ScrollbarOrientation m_orientation;
@@ skipping 20 matching lines @@
     float m_elasticOverscroll;
 
 private:
     bool isScrollbar() const override { return true; }
 
     void invalidate() override { setNeedsPaintInvalidation(); }
     void invalidateRect(const IntRect&) override { setNeedsPaintInvalidation(); }
 
     float scrollableAreaCurrentPos() const;
 
-    void updateThumbPosition();
-    void updateThumbProportion();
-
     bool m_trackNeedsRepaint;
     bool m_thumbNeedsRepaint;
 };
 
 DEFINE_TYPE_CASTS(Scrollbar, Widget, widget, widget->isScrollbar(), widget.isScrollbar());
 
 } // namespace blink
 
 #endif // Scrollbar_h