Fix fixed-pos jitter related to physical pixel snapping.

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 c1bc1a12c4dff940fccb6e5f4c4bbb2e89d79213..88b30bbbf62a62d6bb6b04e822fcf2d7fbe1780b 100644
--- a/cc/trees/layer_tree_host_common.cc
+++ b/cc/trees/layer_tree_host_common.cc
@@ -1518,25 +1518,6 @@ static void CalculateDrawPropertiesInternal(
     combined_transform.Translate(position.x(), position.y());
   }
 
-  gfx::Vector2dF effective_scroll_delta = GetEffectiveScrollDelta(layer);
-  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;
-  }
-
-  // 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;
@@ -1561,6 +1542,30 @@ static void CalculateDrawPropertiesInternal(
           globals.page_scale_factor : 1.f,
       animating_transform_to_screen);
 
+  gfx::Vector2dF effective_scroll_delta = GetEffectiveScrollDelta(layer);
+  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.

[enne (OOO), 2014/03/31 22:45:22]

Can you also mention why you're scaling the offset?  If I understand this bug
correctly, it's because there's a scale earlier in the tree, so the scroll delta
gets multiplied by that scale when applied to children, and so you have to
correct for that here.

[aelias_OOO_until_Jul13, 2014/04/01 06:24:20]

Added a longer comment.

+    effective_scroll_delta -=
+        gfx::ScaleVector2d(current_translation - previous_translation,
+                           1 / combined_transform_scales.x(),

[enne (OOO), 2014/03/31 22:45:22]

style nit: 1.f

[aelias_OOO_until_Jul13, 2014/04/01 06:24:20]

Done.

+                           1 / combined_transform_scales.y());
+  }
+
+  // Apply adjustment from position constraints.
+  ApplyPositionAdjustment(layer,
+                          data_from_ancestor.fixed_container,
+                          data_from_ancestor.scroll_compensation_matrix,
+                          &combined_transform);
+
   // The draw_transform that gets computed below is effectively the layer's
   // draw_transform, unless the layer itself creates a render_surface. In that
   // case, the render_surface re-parents the transforms.