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..c78ad0c3a68432d05a002a440c7372e9b131cbb8 100644
--- a/Source/core/page/scrolling/ScrollingCoordinator.cpp
+++ b/Source/core/page/scrolling/ScrollingCoordinator.cpp
@@ -390,10 +390,18 @@ 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 {
+            DoublePoint scrollPosition(scrollableArea->scrollPositionDouble() - scrollableArea->minimumScrollPosition());
+            IntPoint flooredScrollPosition(flooredIntPoint(scrollPosition));
+            DoublePoint fractionalPart(scrollPosition.x() - flooredScrollPosition.x(), scrollPosition.y() - flooredScrollPosition.y());
+            webLayer->setScrollPositionDouble(scrollPosition);
+            // Blink can only use the integer part of the scroll offset to position elements.
+            // Sends the fractional part of the scroll offset to CC as scroll adjustment for
+            // fixed-position layer.
+            webLayer->setScrollCompensationAdjustment(fractionalPart);
+        }
 
         webLayer->setBounds(scrollableArea->contentsSize());
         bool canScrollX = scrollableArea->userInputScrollable(HorizontalScrollbar);