Fixed position layer counter-scroll with main thread scroll offset fractional part

cc/trees/layer_tree_host_common.cc

Index: cc/trees/layer_tree_host_common.cc
diff --git a/cc/trees/layer_tree_host_common.cc b/cc/trees/layer_tree_host_common.cc
index e12e272653ab7fa2ed335c41ed084711a0da16c0..b05f8ec0e4a335bbad076c29f097c5de73740d27 100644
--- a/cc/trees/layer_tree_host_common.cc
+++ b/cc/trees/layer_tree_host_common.cc
@@ -46,13 +46,21 @@ static void SortLayers(LayerImplList::iterator first,
 
 template <typename LayerType>
 static gfx::Vector2dF GetEffectiveScrollDelta(LayerType* layer) {
-  gfx::Vector2dF scroll_delta = layer->ScrollDelta();
+  // Layer's scroll offset can have an integer part and fractional part.
+  // Due to Blink's limitation, it only counter-scrolls the position-fixed
+  // layer using the integer part of Layer's scroll offset.
+  // CC scrolls the layer using the full scroll offset, so we have to
+  // add the fractional part of the scroll offset to the effective scroll
+  // delta which is used to counter-scroll the position-fixed layer.
+  gfx::Vector2dF scroll_delta =
+      layer->ScrollDelta() + layer->MainScrollOffsetFractionalPart();
   // The scroll parent's scroll delta is the amount we've scrolled on the
   // compositor thread since the commit for this layer tree's source frame.
   // we last reported to the main thread. I.e., it's the discrepancy between
   // a scroll parent's scroll delta and offset, so we must add it here.
   if (layer->scroll_parent())
-    scroll_delta += layer->scroll_parent()->ScrollDelta();
+    scroll_delta += layer->scroll_parent()->ScrollDelta() +
+                    layer->scroll_parent()->MainScrollOffsetFractionalPart();
   return scroll_delta;
 }