cc: Use to_screen transform for scroll offset snapping

cc/trees/layer_tree_host_unittest_scroll.cc

 // Copyright 2012 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.h"
 
 #include "base/location.h"
 #include "base/memory/weak_ptr.h"
 #include "base/single_thread_task_runner.h"
 #include "base/threading/thread_task_runner_handle.h"
@@ skipping 479 matching lines @@
   }
 
   void AfterTest() override {}
 
  private:
   gfx::Vector2dF scroll_amount_;
 };
 
 MULTI_THREAD_TEST_F(LayerTreeHostScrollTestFractionalScroll);
 
+class LayerTreeHostScrollTestScrollSnapping : public LayerTreeHostScrollTest {
+ public:
+  LayerTreeHostScrollTestScrollSnapping() : scroll_amount_(1.75, 0) {}
+
+  void SetupTree() override {
+    LayerTreeHostScrollTest::SetupTree();
+    layer_tree_host()
+        ->outer_viewport_scroll_layer()
+        ->scroll_clip_layer()
+        ->SetForceRenderSurfaceForTesting(true);
+    gfx::Transform translate;
+    translate.Translate(0.25f, 0.f);
+    layer_tree_host()
+        ->outer_viewport_scroll_layer()
+        ->scroll_clip_layer()
+        ->SetTransform(translate);
+    layer_tree_host()->SetPageScaleFactorAndLimits(1.f, 0.1f, 100.f);
+  }
+
+  void BeginTest() override { PostSetNeedsCommitToMainThread(); }
+
+  void DrawLayersOnThread(LayerTreeHostImpl* impl) override {
+    LayerImpl* scroll_layer = impl->OuterViewportScrollLayer();
+    gfx::Transform translate;
+
+    // Check that screen space transform of the scrollable layer is correctly
+    // snapped to integers.
+    switch (impl->active_tree()->source_frame_number()) {
+      case 0:
+        translate.Translate(0, 0);
+        EXPECT_EQ(translate,

[sunxd, 2016/05/17 18:19:45]

I'm not sure if there is a better variable to compare here.

[ajuma, 2016/05/17 19:18:51]

You could compare to the identity (either by comparing to gfx::Transform(), or
by checking if screen_space_transform.IsIdentity is true).

[sunxd, 2016/05/17 19:38:01]

Done.

+                  scroll_layer->draw_properties().screen_space_transform);
+        PostSetNeedsCommitToMainThread();
+        break;
+      case 1:
+        translate.Translate(-2, 0);
+        EXPECT_EQ(translate,
+                  scroll_layer->draw_properties().screen_space_transform);
+        PostSetNeedsCommitToMainThread();
+        break;
+      case 2:
+        translate.Translate(-3, 0);
+        EXPECT_EQ(translate,
+                  scroll_layer->draw_properties().screen_space_transform);
+        EndTest();
+        break;
+    }
+    scroll_layer->ScrollBy(scroll_amount_);
+  }
+
+  void AfterTest() override {}
+
+ private:
+  gfx::Vector2dF scroll_amount_;
+};
+
+MULTI_THREAD_TEST_F(LayerTreeHostScrollTestScrollSnapping);
+
 class LayerTreeHostScrollTestCaseWithChild : public LayerTreeHostScrollTest {
  public:
   LayerTreeHostScrollTestCaseWithChild()
       : initial_offset_(10, 20),
         javascript_scroll_(40, 5),
         scroll_amount_(2, -1),
         num_scrolls_(0) {}
 
   void SetupTree() override {
     layer_tree_host()->SetDeviceScaleFactor(device_scale_factor_);
@@ skipping 941 matching lines @@
   RunTest(CompositorMode::THREADED, false);
 }
 
 TEST_F(LayerTreeHostScrollTestLayerStructureChange, ScrollDestroyWholeTree) {
   scroll_destroy_whole_tree_ = true;
   RunTest(CompositorMode::THREADED, false);
 }
 
 }  // namespace
 }  // namespace cc