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

cc/trees/layer_tree_host_common.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/layer_tree_host_common.h"
 
 #include <algorithm>
 
 #include "base/debug/trace_event.h"
 #include "cc/base/math_util.h"
@@ skipping 28 matching lines @@
 static void SortLayers(LayerImplList::iterator first,
                        LayerImplList::iterator end,
                        LayerSorter* layer_sorter) {
   DCHECK(layer_sorter);
   TRACE_EVENT0("cc", "LayerTreeHostCommon::SortLayers");
   layer_sorter->Sort(first, end);
 }
 
 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;
 }
 
 template <typename LayerType>
 static gfx::ScrollOffset GetEffectiveTotalScrollOffset(LayerType* layer) {
   gfx::ScrollOffset offset = layer->TotalScrollOffset();
   // The scroll parent's total scroll offset (scroll offset + scroll delta)
   // can't be used because its scroll offset has already been applied to the
   // scroll children's positions by the main thread layer positioning code.
   if (layer->scroll_parent())
@@ skipping 2498 matching lines @@
       inputs->current_render_surface_layer_list_id);
 
   // The dummy layer list should not have been used.
   DCHECK_EQ(0u, dummy_layer_list.size());
   // A root layer render_surface should always exist after
   // CalculateDrawProperties.
   DCHECK(inputs->root_layer->render_surface());
 }
 
 }  // namespace cc