Pass integer scroll position to CC if frameView has non-layered viewport constrained objects

Source/core/page/scrolling/ScrollingCoordinator.cpp

 /*
  * Copyright (C) 2011 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 337 matching lines @@
         setupScrollbarLayer(scrollbarGraphicsLayer, scrollbarLayer, scrollLayer, containerLayer);
 
         // Root layer non-overlay scrollbars should be marked opaque to disable
         // blending.
         bool isOpaqueScrollbar = !scrollbar->isOverlayScrollbar();
         scrollbarGraphicsLayer->setContentsOpaque(isMainFrame && isOpaqueScrollbar);
     } else
         removeWebScrollbarLayer(scrollableArea, orientation);
 }
 
-bool ScrollingCoordinator::scrollableAreaScrollLayerDidChange(ScrollableArea* scrollableArea)
+bool ScrollingCoordinator::scrollableAreaScrollLayerDidChange(ScrollableArea* scrollableArea, bool hasVisibleViewportConstrainedObjects)
 {
     GraphicsLayer* scrollLayer = scrollableArea->layerForScrolling();
 
     if (scrollLayer) {
         ASSERT(m_page);
         scrollLayer->setScrollableArea(scrollableArea, isForViewport(scrollableArea));
     }
 
     WebLayer* webLayer = toWebLayer(scrollableArea->layerForScrolling());
     WebLayer* containerLayer = toWebLayer(scrollableArea->layerForContainer());
     if (webLayer) {
         webLayer->setScrollClipLayer(containerLayer);
-        webLayer->setScrollPositionDouble(DoublePoint(scrollableArea->scrollPositionDouble() - scrollableArea->minimumScrollPosition()));
+        // Viewport constrained objects, e.g. fixed position elements, are
+        // positioned in Blink using integer coordinates. In that case, we
+        // don't want to set the WebLayer's scroll position at fractional
+        // precision otherwise the WebLayer's position after snapping to
+        // device pixel can be off with regard to fixed position elements.
+        // FIXME: fixed position elements which have their own compositing
+        // layer can actually be positioned at device pixel boundary in
+        // chromium compositor, so this should really be conditioned on
+        // non-composited fixed position elements. crbug.com/414283.
+        if (hasVisibleViewportConstrainedObjects) {

[Rick Byers, 2014/12/05 19:01:41]

Rather than taking this bit in from our caller (which computes it independently
in FrameView), can we just test m_lastMainThreadScrollingReasons for the
HasNonLayerViewportConstrainedObjects flag?  We really should be able to share
this logic somehow.  I don't know if there are potential timing issues where
m_lastMainThreadScrollingReasons is out of date.

+            webLayer->setScrollPositionDouble(DoublePoint(scrollableArea->scrollPosition() - scrollableArea->minimumScrollPosition()));
+        } else {
+            webLayer->setScrollPositionDouble(DoublePoint(scrollableArea->scrollPositionDouble() - scrollableArea->minimumScrollPosition()));
+        }
         webLayer->setBounds(scrollableArea->contentsSize());
         bool canScrollX = scrollableArea->userInputScrollable(HorizontalScrollbar);
         bool canScrollY = scrollableArea->userInputScrollable(VerticalScrollbar);
         webLayer->setUserScrollable(canScrollX, canScrollY);
     }
     if (WebScrollbarLayer* scrollbarLayer = getWebScrollbarLayer(scrollableArea, HorizontalScrollbar)) {
         GraphicsLayer* horizontalScrollbarLayer = scrollableArea->layerForHorizontalScrollbar();
         if (horizontalScrollbarLayer)
             setupScrollbarLayer(horizontalScrollbarLayer, scrollbarLayer, webLayer, containerLayer);
     }
@@ skipping 610 matching lines @@
     bool frameIsScrollable = frameView && frameView->isScrollable();
     if (frameIsScrollable != m_wasFrameScrollable)
         return true;
 
     if (WebLayer* scrollLayer = frameView ? toWebLayer(frameView->layerForScrolling()) : nullptr)
         return blink::WebSize(frameView->contentsSize()) != scrollLayer->bounds();
     return false;
 }
 
 } // namespace blink