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

cc/layers/layer.cc

Index: cc/layers/layer.cc
diff --git a/cc/layers/layer.cc b/cc/layers/layer.cc
index feb23fa8d84f4e800c135e606335a47d103d03ff..f94d23393c93b5624b7776a4d2b216feba605849 100644
--- a/cc/layers/layer.cc
+++ b/cc/layers/layer.cc
@@ -672,6 +672,24 @@ void Layer::SetScrollOffset(const gfx::ScrollOffset& scroll_offset) {
   SetNeedsCommit();
 }
 
+void Layer::SetScrollOffsetFractionalPart(

[aelias_OOO_until_Jul13, 2015/01/28 22:34:16]

Let's not mutate scroll_offset_ in this method.  We can just do something very
simple here:

void Layer::SetScrollOffsetFractionalPart(...) {
  if (scroll_offset_fractional_part_ == scroll_offset_fractional_part)
    return;
  scroll_offset_fractional_part_ = scroll_offset_fractional_part;
  SetNeedsCommit();
}

gfx::Vector2dF Layer::MainScrollOffsetFractionalPart() const {
  return scroll_offset_fractional_part_;
}

[Yufeng Shen (Slow to review), 2015/01/28 22:49:27]

Done.

+    const gfx::Vector2dF& scroll_offset_fractional_part) {
+  gfx::ScrollOffset floored_scroll_offset(
+      gfx::ScrollOffsetToFlooredVector2d(scroll_offset_));
+  gfx::ScrollOffset new_scroll_offset =
+      ScrollOffsetWithDelta(floored_scroll_offset,
+                            scroll_offset_fractional_part);
+  if (new_scroll_offset == scroll_offset_)
+    return;
+  scroll_offset_ = new_scroll_offset;
+  SetNeedsCommit();
+}
+
+gfx::Vector2dF Layer::MainScrollOffsetFractionalPart() const {
+  return scroll_offset_.DeltaFrom(gfx::ScrollOffset(
+      gfx::ScrollOffsetToFlooredVector2d(scroll_offset_)));
+}
+
 void Layer::SetScrollOffsetFromImplSide(
     const gfx::ScrollOffset& scroll_offset) {
   DCHECK(IsPropertyChangeAllowed());
@@ -981,6 +999,7 @@ void Layer::PushPropertiesTo(LayerImpl* layer) {
     }
     layer->SetSentScrollDelta(gfx::Vector2dF());
   }
+  layer->SetMainScrollOffsetFractionalPart(MainScrollOffsetFractionalPart());
 
   // Wrap the copy_requests_ in a PostTask to the main thread.
   ScopedPtrVector<CopyOutputRequest> main_thread_copy_requests;