Keep expanded if mouse moves off of scrollbar while dragging

cc/trees/layer_tree_host_impl.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_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 222 matching lines @@
       resourceless_software_draw_(false),
       animation_host_(std::move(animation_host)),
       rendering_stats_instrumentation_(rendering_stats_instrumentation),
       micro_benchmark_controller_(this),
       shared_bitmap_manager_(shared_bitmap_manager),
       gpu_memory_buffer_manager_(gpu_memory_buffer_manager),
       task_graph_runner_(task_graph_runner),
       id_(id),
       requires_high_res_to_draw_(false),
       is_likely_to_require_a_draw_(false),
-      mutator_(nullptr) {
+      mutator_(nullptr),
+      captured_scrollbar_layer_id_(Layer::INVALID_ID) {
   DCHECK(animation_host_);
   animation_host_->SetMutatorHostClient(this);
 
   DCHECK(task_runner_provider_->IsImplThread());
   DidVisibilityChange(this, visible_);
 
   SetDebugState(settings.initial_debug_state);
 
   // LTHI always has an active tree.
   active_tree_ = base::MakeUnique<LayerTreeImpl>(
@@ skipping 3008 matching lines @@
 
   gfx::Rect layer_impl_bounds(layer_impl->bounds());
 
   gfx::RectF device_viewport_layer_impl_bounds = MathUtil::MapClippedRect(
       layer_impl->ScreenSpaceTransform(), gfx::RectF(layer_impl_bounds));
 
   return device_viewport_layer_impl_bounds.ManhattanDistanceToPoint(
       device_viewport_point);
 }
 
+void LayerTreeHostImpl::MouseDownAt(const gfx::Point& viewport_point) {
+  if (scroll_layer_id_when_mouse_over_scrollbar_ != Layer::INVALID_ID) {

[bokan, 2016/09/23 00:39:12]

Nit: Negate the condition and early return instead.

+    captured_scrollbar_layer_id_ = scroll_layer_id_when_mouse_over_scrollbar_;
+    ScrollbarAnimationController* animation_controller =
+        ScrollbarAnimationControllerForId(captured_scrollbar_layer_id_);
+    if (animation_controller)
+      animation_controller->DidCaptureScrollbarBegin();
+  }
+}
+
+void LayerTreeHostImpl::MouseUp() {
+  if (captured_scrollbar_layer_id_ != Layer::INVALID_ID) {

[bokan, 2016/09/23 00:39:12]

Ditto here

+    ScrollbarAnimationController* animation_controller =
+        ScrollbarAnimationControllerForId(captured_scrollbar_layer_id_);
+    if (animation_controller)
+      animation_controller->DidCaptureScrollbarEnd();
+
+    captured_scrollbar_layer_id_ = Layer::INVALID_ID;
+  }
+}
+
 void LayerTreeHostImpl::MouseMoveAt(const gfx::Point& viewport_point) {
   gfx::PointF device_viewport_point = gfx::ScalePoint(
       gfx::PointF(viewport_point), active_tree_->device_scale_factor());
   LayerImpl* layer_impl =
       active_tree_->FindLayerThatIsHitByPoint(device_viewport_point);
   HandleMouseOverScrollbar(layer_impl);
   if (scroll_layer_id_when_mouse_over_scrollbar_ != Layer::INVALID_ID)
     return;
 
   bool scroll_on_main_thread = false;
@@ skipping 915 matching lines @@
   if (is_visible) {
     worker_context_visibility_ =
         worker_context->CacheController()->ClientBecameVisible();
   } else {
     worker_context->CacheController()->ClientBecameNotVisible(
         std::move(worker_context_visibility_));
   }
 }
 
 }  // namespace cc