Fix scroll position adjustment for fixed-position layers

cc/trees/property_tree.cc

 // Copyright 2014 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 <set>
 #include <vector>
 
 #include "base/logging.h"
 #include "cc/base/math_util.h"
 #include "cc/trees/property_tree.h"
@@ skipping 298 matching lines @@
   // Scroll snapping must be done in target space (the pixels we care about).
   // This means we effectively snap the target space transform. If TT is the
   // target space transform and TT' is TT with its translation components
   // rounded, then what we're after is the scroll delta X, where TT * X = TT'.
   // I.e., we want a transform that will realize our scroll snap. It follows
   // that X = TT^-1 * TT'. We cache TT and TT^-1 to make this more efficient.
   gfx::Transform rounded = node->data.to_target;
   rounded.RoundTranslationComponents();
   gfx::Transform delta = node->data.from_target;
   delta *= rounded;
-  gfx::Transform inverse_delta(gfx::Transform::kSkipInitialization);
-  bool invertible_delta = delta.GetInverse(&inverse_delta);
 
-  // The delta should be a translation, modulo floating point error, and should
-  // therefore be invertible.
-  DCHECK(invertible_delta);
+  DCHECK(delta.IsIdentityOr2DTranslation());
+
+  gfx::Vector2dF translation = delta.To2dTranslation();
 
   // Now that we have our scroll delta, we must apply it to each of our
   // combined, to/from matrices.
-  node->data.to_parent.PreconcatTransform(delta);
-  node->data.to_target.PreconcatTransform(delta);
-  node->data.from_target.ConcatTransform(inverse_delta);
-  node->data.to_screen.PreconcatTransform(delta);
-  node->data.from_screen.ConcatTransform(inverse_delta);
+  node->data.to_parent.Translate(translation.x(), translation.y());
+  node->data.to_target.Translate(translation.x(), translation.y());
+  node->data.from_target.matrix().postTranslate(-translation.x(),
+                                                -translation.y(), 0);
+  node->data.to_screen.Translate(translation.x(), translation.y());
+  node->data.from_screen.matrix().postTranslate(-translation.x(),
+                                                -translation.y(), 0);
+
+  node->data.scroll_snap = translation;
 }
 
 }  // namespace cc