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..ad33dfbe3f723d48846554b486aa18b6cae19400 100644
--- a/Source/core/page/scrolling/ScrollingCoordinator.cpp
+++ b/Source/core/page/scrolling/ScrollingCoordinator.cpp
@@ -390,10 +390,21 @@ 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 integer and 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 these separate
+            // calls.
+            DoublePoint scrollPosition(scrollableArea->scrollPositionDouble() - scrollableArea->minimumScrollPosition());
+            IntPoint flooredScrollPosition(flooredIntPoint(scrollPosition));
+            DoublePoint fractionalPart(scrollPosition.x() - flooredScrollPosition.x(), scrollPosition.y() - flooredScrollPosition.y());
+            webLayer->setScrollPositionDouble(DoublePoint(flooredScrollPosition));

[aelias_OOO_until_Jul13, 2015/01/28 22:31:34]

Let's send the full nonfloored position here.  The idea is that in CC we would
never need to reason about floored stuff in the normal scroll offset codepaths,
it can always assume scroll_offset_ contains the full offset from now on.

[Yufeng Shen (Slow to review), 2015/01/28 22:49:33]

Done.

[Rick Byers, 2015/01/29 20:55:07]

Just saw this - sorry.  Maybe I just need an concise API description in the
comments - clearly the semantics aren't as simple as "one API for the integer
portion, one API for the fractional portion".

+            webLayer->setScrollPositionFractionalPart(fractionalPart);
+        }
 
         webLayer->setBounds(scrollableArea->contentsSize());
         bool canScrollX = scrollableArea->userInputScrollable(HorizontalScrollbar);