Update matrix distance metric to account for 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/trace_event/trace_event.h"
 #include "cc/base/math_util.h"
@@ skipping 2447 matching lines @@
   // TODO(vollick): This tolerance should be lower: crbug.com/471786
   static const int tolerance = 2;
   return std::abs(r1.x() - r2.x()) <= tolerance &&
          std::abs(r1.y() - r2.y()) <= tolerance &&
          std::abs(r1.right() - r2.right()) <= tolerance &&
          std::abs(r1.bottom() - r2.bottom()) <= tolerance;
 }
 
 static bool ApproximatelyEqual(const gfx::Transform& a,
                                const gfx::Transform& b) {
-  static const float tolerance = 0.1f;
-  return gfx::MatrixDistance(a, b) <= tolerance;
+  static const float component_tolerance = 0.1f;
+
+  // We may have a larger discrepancy in the scroll components due to snapping
+  // (floating point error might round the other way).
+  static const float translation_tolerance = 1.f;
+
+  for (int row = 0; row < 4; row++) {
+    for (int col = 0; col < 4; col++) {
+      static const float delta =
+          std::abs(a.matrix().get(row, col) - b.matrix().get(row, col));
+      const float tolerance =
+          col == 3 && row < 3 ? translation_tolerance : component_tolerance;
+      if (delta > tolerance)
+        return false;
+    }
+  }
+
+  return true;
 }
 
 void LayerTreeHostCommon::CalculateDrawProperties(
     CalcDrawPropsMainInputs* inputs) {
   UpdateRenderSurfaces(inputs->root_layer,
                        inputs->can_render_to_separate_surface, gfx::Transform(),
                        false);
   LayerList dummy_layer_list;
   SubtreeGlobals<Layer> globals;
   DataForRecursion<Layer> data_for_recursion;
@@ skipping 91 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