Fix smooth scroll jump when switching scroll handling between MT and CC

cc/layers/layer_unittest.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/layers/layer.h"
 
 #include <stddef.h>
 
 #include "base/thread_task_runner_handle.h"
 #include "cc/animation/animation_host.h"
@@ skipping 910 matching lines @@
   EXPECT_FALSE(test_layer->NeedsDisplayForTesting());
 
   // Case 4: SetNeedsDisplay() with a non-drawable layer
   EXPECT_SET_NEEDS_COMMIT(1, test_layer->SetIsDrawable(false));
   test_layer->ResetNeedsDisplayForTesting();
   EXPECT_FALSE(test_layer->NeedsDisplayForTesting());
   EXPECT_SET_NEEDS_UPDATE(0, test_layer->SetNeedsDisplayRect(dirty1));
   EXPECT_TRUE(test_layer->NeedsDisplayForTesting());
 }
 
+TEST_F(LayerTest, TestSettingMainThreadScrollingReason) {
+  scoped_refptr<Layer> test_layer = Layer::Create(layer_settings_);
+  EXPECT_SET_NEEDS_FULL_TREE_SYNC(1,
+                                  layer_tree_host_->SetRootLayer(test_layer));
+  EXPECT_SET_NEEDS_COMMIT(1, test_layer->SetIsDrawable(true));
+
+  // sanity check of initial test condition
+  EXPECT_FALSE(test_layer->NeedsDisplayForTesting());
+
+  uint32_t reasons = 0, reasons_to_clear = 0, reasons_after_clearing = 0;
+  reasons |= MainThreadScrollingReason::kEventHandlers;
+  reasons |= MainThreadScrollingReason::kContinuingMainThreadScroll;
+  reasons |= MainThreadScrollingReason::kScrollbarScrolling;
+
+  reasons_to_clear |= MainThreadScrollingReason::kContinuingMainThreadScroll;
+  reasons_to_clear |= MainThreadScrollingReason::kThreadedScrollingDisabled;
+
+  reasons_after_clearing |= MainThreadScrollingReason::kEventHandlers;
+  reasons_after_clearing |= MainThreadScrollingReason::kScrollbarScrolling;
+
+  // Check that the reasons are added correctly.
+  EXPECT_SET_NEEDS_COMMIT(1, test_layer->AddMainThreadScrollingReasons(
+                                 MainThreadScrollingReason::kEventHandlers));
+  EXPECT_SET_NEEDS_COMMIT(
+      1, test_layer->AddMainThreadScrollingReasons(
+             MainThreadScrollingReason::kContinuingMainThreadScroll));
+  EXPECT_SET_NEEDS_COMMIT(1,
+                          test_layer->AddMainThreadScrollingReasons(
+                              MainThreadScrollingReason::kScrollbarScrolling));
+  EXPECT_EQ(reasons, test_layer->main_thread_scrolling_reasons());
+
+  // Check that the reasons can be selectively cleared.
+  EXPECT_SET_NEEDS_COMMIT(
+      1, test_layer->ClearMainThreadScrollingReasons(reasons_to_clear));
+  EXPECT_EQ(reasons_after_clearing,
+            test_layer->main_thread_scrolling_reasons());
+
+  // Check that all reasons can be cleared.
+  EXPECT_SET_NEEDS_COMMIT(1, test_layer->ClearMainThreadScrollingReasons());
+  EXPECT_EQ((uint32_t)0, test_layer->main_thread_scrolling_reasons());
+}
+
 TEST_F(LayerTest, CheckPropertyChangeCausesCorrectBehavior) {
   scoped_refptr<Layer> test_layer = Layer::Create(layer_settings_);
   EXPECT_SET_NEEDS_FULL_TREE_SYNC(
       1, layer_tree_host_->SetRootLayer(test_layer));
   EXPECT_SET_NEEDS_COMMIT(1, test_layer->SetIsDrawable(true));
 
   scoped_refptr<Layer> dummy_layer1 = Layer::Create(layer_settings_);
   scoped_refptr<Layer> dummy_layer2 = Layer::Create(layer_settings_);
 
   // sanity check of initial test condition
@@ skipping 1177 matching lines @@
   EXPECT_EQ(MutableProperty::kNone, impl_layer->mutable_properties());
 
   test_layer->PushPropertiesTo(impl_layer.get());
 
   EXPECT_EQ(2lu, impl_layer->element_id());
   EXPECT_EQ(MutableProperty::kTransform, impl_layer->mutable_properties());
 }
 
 }  // namespace
 }  // namespace cc