Stop scrolling when the scrolling node disappears in the new tree

We need to reset the scrolling status (especially LayerTreeHostImpl::
did_lock_scrolling_layer_) when activating the pending tree in which
the currently scrolling node no longer exists.

Previously the situation caused DCHECK failure.

Change-Id: I23b9105ded1767ee27fa37fb75f67c13e81d5699
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1882301Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710530}

cc/trees/layer_tree_host_impl.cc

@@ -2974,6 +2974,10 @@ void LayerTreeHostImpl::ActivateSyncTree() {
 
     active_tree_->lifecycle().AdvanceTo(LayerTreeLifecycle::kNotSyncing);
 
+    // The previous scrolling node no longer exists in the new tree.
+    if (!active_tree_->CurrentlyScrollingNode())
+      ClearCurrentlyScrollingNode();
+
     // Now that we've synced everything from the pending tree to the active
     // tree, rename the pending tree the recycle tree so we can reuse it on the
     // next sync.

cc/trees/layer_tree_host_impl.h

@@ -806,6 +806,10 @@ class CC_EXPORT LayerTreeHostImpl : public InputHandler,
     return paint_worklet_tracker_;
   }
 
+  bool did_lock_scrolling_layer_for_testing() const {
+    return did_lock_scrolling_layer_;
+  }
+
  protected:
   LayerTreeHostImpl(
       const LayerTreeSettings& settings,

cc/trees/layer_tree_host_impl_unittest.cc

@@ -1147,6 +1147,34 @@ TEST_F(LayerTreeHostImplTest, ReplaceTreeWhileScrolling) {
                                  scroll_delta));
 }
 
+TEST_F(LayerTreeHostImplTest, ActivateTreeScrollingNodeDisappeared) {
+  SetupViewportLayersOuterScrolls(gfx::Size(100, 100), gfx::Size(1000, 1000));
+
+  auto status = host_impl_->ScrollBegin(BeginState(gfx::Point(30, 30)).get(),
+                                        InputHandler::WHEEL);
+  EXPECT_EQ(InputHandler::SCROLL_ON_IMPL_THREAD, status.thread);
+  EXPECT_EQ(MainThreadScrollingReason::kNotScrollingOnMain,
+            status.main_thread_scrolling_reasons);
+  host_impl_->ScrollBy(UpdateState(gfx::Point(), gfx::Vector2d(0, 10)).get());
+  EXPECT_TRUE(host_impl_->active_tree()->CurrentlyScrollingNode());
+  EXPECT_TRUE(host_impl_->did_lock_scrolling_layer_for_testing());
+  EXPECT_TRUE(host_impl_->IsActivelyScrolling());
+
+  // Create the pending tree containing only the root layer.
+  CreatePendingTree();
+  PropertyTrees pending_property_trees;
+  pending_property_trees.sequence_number =
+      host_impl_->active_tree()->property_trees()->sequence_number + 1;
+  host_impl_->pending_tree()->SetPropertyTrees(&pending_property_trees);
+  SetupRootLayer<LayerImpl>(host_impl_->pending_tree(), gfx::Size(100, 100));
+  host_impl_->ActivateSyncTree();
+
+  // The scroll should stop.
+  EXPECT_FALSE(host_impl_->active_tree()->CurrentlyScrollingNode());
+  EXPECT_FALSE(host_impl_->did_lock_scrolling_layer_for_testing());
+  EXPECT_FALSE(host_impl_->IsActivelyScrolling());
+}
+
 TEST_F(LayerTreeHostImplTest, ScrollBlocksOnWheelEventHandlers) {
   SetupViewportLayersInnerScrolls(gfx::Size(50, 50), gfx::Size(100, 100));
   auto* scroll = InnerViewportScrollLayer();