Make ScrollbarThemeAura selectively invalidate scrollbar parts.

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

Index: third_party/WebKit/Source/platform/scroll/Scrollbar.h
diff --git a/third_party/WebKit/Source/platform/scroll/Scrollbar.h b/third_party/WebKit/Source/platform/scroll/Scrollbar.h
index 0cf202f7f0ed9c1afe03aeacaca0b12e4176bfa1..237631aee0ab2afe09b667386c5075465b782936 100644
--- a/third_party/WebKit/Source/platform/scroll/Scrollbar.h
+++ b/third_party/WebKit/Source/platform/scroll/Scrollbar.h
@@ -94,7 +94,15 @@ public:
     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 is painted in a separate layer.

[skobes, 2015/12/28 17:48:39]

Technically the thumb is never a separate layer.  Perhaps this should read "the
scrollbar is unaware of whether its layer can move the thumb without
repainting"?

[jbroman, 2015/12/29 16:45:14]

reworded to "...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; }
@@ -141,6 +149,7 @@ public:
     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; }
@@ -154,7 +163,9 @@ public:
     // 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();
@@ -166,7 +177,6 @@ public:
 protected:
     Scrollbar(ScrollableArea*, ScrollbarOrientation, ScrollbarControlSize, ScrollbarTheme* = 0);
 
-    void updateThumb();
 
     void autoscrollTimerFired(Timer<Scrollbar>*);
     void startTimerIfNeeded(double delay);
@@ -207,9 +217,6 @@ private:
 
     float scrollableAreaCurrentPos() const;
 
-    void updateThumbPosition();
-    void updateThumbProportion();
-
     bool m_trackNeedsRepaint;
     bool m_thumbNeedsRepaint;
 };