Let synchronized painting generate correct paint invalidation rects

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

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 202 matching lines @@
         return;
 
     if (scrollbar == verticalScrollbar())
         scrollRect.move(verticalScrollbarStart(0, box().size().width()), box().borderTop());
     else
         scrollRect.move(horizontalScrollbarStart(0), box().size().height() - box().borderBottom() - scrollbar->height());
 
     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));
     }
 }
 
 void PaintLayerScrollableArea::invalidateScrollCornerRect(const IntRect& rect)
 {
     ASSERT(!layerForScrollCorner());
 
     if (m_scrollCorner) {
         // FIXME: We should not allow paint invalidation out of paint invalidation state. crbug.com/457415
         DisablePaintInvalidationStateAsserts disabler;
@@ skipping 1330 matching lines @@
 }
 
 DEFINE_TRACE(PaintLayerScrollableArea::ScrollbarManager)
 {
     visitor->trace(m_scrollableArea);
     visitor->trace(m_hBar);
     visitor->trace(m_vBar);
 }
 
 } // namespace blink