Fix fixed-pos jitter related to physical pixel snapping.

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 1511 matching lines @@
   if (!animating_transform_to_target && layer->scrollable() &&
       combined_transform.IsScaleOrTranslation()) {
     // Align the scrollable layer's position to screen space pixels to avoid
     // blurriness.  To avoid side-effects, do this only if the transform is
     // simple.
     gfx::Vector2dF previous_translation = combined_transform.To2dTranslation();
     RoundTranslationComponents(&combined_transform);
     gfx::Vector2dF current_translation = combined_transform.To2dTranslation();
 
     // This rounding changes the scroll delta, and so must be included
-    // in the scroll compensation matrix.
-    effective_scroll_delta -= current_translation - previous_translation;
+    // in the scroll compensation matrix.  The scaling converts from physical
+    // coordinates to the scroll delta's CSS coordinates (using the parent
+    // matrix instead of combined transform since scrolling is applied before
+    // the layer's transform).  For example, if we have a total scale factor of
+    // 3.0, then 1 physical pixel is only 1/3 of a CSS pixel.
+    gfx::Vector2dF parent_scales = MathUtil::ComputeTransform2dScaleComponents(
+        data_from_ancestor.parent_matrix, 1.f);
+    effective_scroll_delta -=
+        gfx::ScaleVector2d(current_translation - previous_translation,
+                           1.f / parent_scales.x(),
+                           1.f / parent_scales.y());
   }
 
   // Apply adjustment from position constraints.
   ApplyPositionAdjustment(layer, data_from_ancestor.fixed_container,
       data_from_ancestor.scroll_compensation_matrix, &combined_transform);
 
   // Compute the 2d scale components of the transform hierarchy up to the target
   // surface. From there, we can decide on a contents scale for the layer.
   float layer_scale_factors = globals.device_scale_factor;
   if (data_from_ancestor.in_subtree_of_page_scale_application_layer)
@@ skipping 853 matching lines @@
   // At this point, we think the point does hit the touch event handler region
   // on the layer, but we need to walk up the parents to ensure that the layer
   // was not clipped in such a way that the hit point actually should not hit
   // the layer.
   if (PointIsClippedBySurfaceOrClipRect(screen_space_point, layer_impl))
     return false;
 
   return true;
 }
 }  // namespace cc