Add a WebLayer:setScrollCompensationAdjustment between Blink and CC

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 372 matching lines @@
 
     WebLayer* webLayer = toWebLayer(scrollableArea->layerForScrolling());
     WebLayer* containerLayer = toWebLayer(scrollableArea->layerForContainer());
     if (webLayer) {
         webLayer->setScrollClipLayer(containerLayer);
         // Non-layered 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.
-        if (m_lastMainThreadScrollingReasons & ScrollingCoordinator::HasNonLayerViewportConstrainedObjects)
+        if (m_lastMainThreadScrollingReasons & ScrollingCoordinator::HasNonLayerViewportConstrainedObjects) {
             webLayer->setScrollPositionDouble(DoublePoint(scrollableArea->scrollPosition() - scrollableArea->minimumScrollPosition()));
-        else
-            webLayer->setScrollPositionDouble(DoublePoint(scrollableArea->scrollPositionDouble() - scrollableArea->minimumScrollPosition()));
+        } else {
+            // Blink can only use the integer part of the scroll offset to position elements.
+            // So it sends the fractional part of the scroll offset to CC separately to be
+            // clear that CC needs to take special care of the fractional part, e.g.
+            // compensating for fixed-position layer's position. Once Blink can fully position
+            // elements at fractional boundary, we can get rid of this call.
+            DoublePoint scrollPosition(scrollableArea->scrollPositionDouble() - scrollableArea->minimumScrollPosition());
+            IntPoint flooredScrollPosition(flooredIntPoint(scrollPosition));
+            DoublePoint fractionalPart(scrollPosition.x() - flooredScrollPosition.x(), scrollPosition.y() - flooredScrollPosition.y());
+            webLayer->setScrollPositionDouble(scrollPosition);

[Rick Byers, 2015/01/29 20:53:13]

You don't want flooredScrollPosition here?  If the API is to always pass the the
full double, then I guess I don't understand why CC doesn't just pull the
fractional part off itself rather than introducing a new API that's always
redundant?

+            webLayer->setScrollPositionFractionalPart(fractionalPart);
+        }
 
         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 627 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