Let synchronized painting generate correct paint invalidation rects

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

Index: third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
index e2b56bff44a3626f445b63f3687f7c2846d42391..52b96b909fac07d8b8e2e44d75954c8a2128e766 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -220,21 +220,18 @@ void PaintLayerScrollableArea::invalidateScrollbarRect(Scrollbar* scrollbar, con
     if (scrollRect.isEmpty())
         return;
 
-    box().invalidateDisplayItemClient(*scrollbar);
-
     LayoutRect paintInvalidationRect = LayoutRect(scrollRect);
     box().flipForWritingMode(paintInvalidationRect);
 
     IntRect intRect = pixelSnappedIntRect(paintInvalidationRect);
 
     if (box().frameView()->isInPerformLayout()) {
+        box().invalidateDisplayItemClient(*scrollbar);

[chrishtr, 2015/10/22 00:02:41]

Why this change?

[Xianzhu, 2015/10/22 01:17:04]

See the comment for old code line 237.

         addScrollbarDamage(scrollbar, intRect);
     } else {
         // FIXME: We should not allow paint invalidation out of paint invalidation state. crbug.com/457415
         DisablePaintInvalidationStateAsserts disabler;
-        // We have invalidated the displayItemClient of the scrollbar, but for now we still need to
-        // invalidate the rectangles to trigger repaints.
-        box().invalidatePaintRectangleNotInvalidatingDisplayItemClients(LayoutRect(intRect));

[chrishtr, 2015/10/22 00:02:41]

Why did the old code avoid invalidating display item clients?

[Xianzhu, 2015/10/22 01:17:04]

Because we had invalidated display item clients above. Now we output
invalidation rects along with invalidations of display item clients, so must
change this otherwise we'll lose this invalidation rect.

+        box().invalidatePaintRectangleForDisplayItemClient(*scrollbar, LayoutRect(intRect));
     }
 }