Add a WebLayer:setScrollCompensationAdjustment between Blink and CC

Source/core/page/scrolling/ScrollingCoordinator.cpp

Index: Source/core/page/scrolling/ScrollingCoordinator.cpp
diff --git a/Source/core/page/scrolling/ScrollingCoordinator.cpp b/Source/core/page/scrolling/ScrollingCoordinator.cpp
index 9ef7d6404d86bb4d5e07d3449ddab67b5d2d70c6..0298446ffbbea595f0d6fa5a1203055e738cf312 100644
--- a/Source/core/page/scrolling/ScrollingCoordinator.cpp
+++ b/Source/core/page/scrolling/ScrollingCoordinator.cpp
@@ -390,10 +390,20 @@ bool ScrollingCoordinator::scrollableAreaScrollLayerDidChange(ScrollableArea* sc
         // 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);