Prevent overlay scrollbars expand or hover together

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 1640 matching lines @@
 
   // Collect all resource ids in the render passes into a single array.
   ResourceProvider::ResourceIdArray resources;
   for (const auto& render_pass : frame->render_passes) {
     for (auto* quad : render_pass->quad_list) {
       for (ResourceId resource_id : quad->resources)
         resources.push_back(resource_id);
     }
   }
 
-
   CompositorFrame compositor_frame;
   compositor_frame.metadata = std::move(metadata);
   resource_provider_->PrepareSendToParent(resources,
                                           &compositor_frame.resource_list);
   compositor_frame.render_pass_list = std::move(frame->render_passes);
   compositor_frame_sink_->SubmitCompositorFrame(std::move(compositor_frame));
 
   // The next frame should start by assuming nothing has changed, and changes
   // are noted as they occur.
   // TODO(boliu): If we did a temporary software renderer frame, propogate the
@@ skipping 1549 matching lines @@
 }
 
 void LayerTreeHostImpl::MouseUp() {
   ScrollbarAnimationController* animation_controller =
       ScrollbarAnimationControllerForId(scroll_layer_id_mouse_currently_over_);
   if (animation_controller)
     animation_controller->DidMouseUp();
 }
 
 void LayerTreeHostImpl::MouseMoveAt(const gfx::Point& viewport_point) {
-  float distance_to_scrollbar = std::numeric_limits<float>::max();
   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);
 
   // Check if mouse is over a scrollbar or not.
   // TODO(sahel): get rid of this extera checking when
   // FindScrollLayerForDeviceViewportPoint finds the proper layer for
   // scrolling on main thread, as well.
   int new_id = Layer::INVALID_ID;
   if (layer_impl && layer_impl->ToScrollbarLayer())
     new_id = layer_impl->ToScrollbarLayer()->ScrollLayerId();
-  if (new_id != Layer::INVALID_ID) {
-    // Mouse over a scrollbar.
-    distance_to_scrollbar = 0;
-  } else {
+  if (new_id == Layer::INVALID_ID) {
     bool scroll_on_main_thread = false;
     uint32_t main_thread_scrolling_reasons;
     LayerImpl* scroll_layer_impl = FindScrollLayerForDeviceViewportPoint(
         device_viewport_point, InputHandler::TOUCHSCREEN, layer_impl,
         &scroll_on_main_thread, &main_thread_scrolling_reasons);
 
     // Scrollbars for the viewport are registered with the outer viewport layer.
     if (scroll_layer_impl == InnerViewportScrollLayer())
       scroll_layer_impl = OuterViewportScrollLayer();
 
     new_id = scroll_layer_impl ? scroll_layer_impl->id() : Layer::INVALID_ID;
   }
 
   if (new_id != scroll_layer_id_mouse_currently_over_) {
     ScrollbarAnimationController* old_animation_controller =
         ScrollbarAnimationControllerForId(
             scroll_layer_id_mouse_currently_over_);
     if (old_animation_controller) {
       old_animation_controller->DidMouseLeave();
     }
     scroll_layer_id_mouse_currently_over_ = new_id;
   }
 
   ScrollbarAnimationController* new_animation_controller =
       ScrollbarAnimationControllerForId(new_id);
   if (!new_animation_controller)
     return;
 
-  for (ScrollbarLayerImplBase* scrollbar : ScrollbarsFor(new_id))
-    distance_to_scrollbar =
-        std::min(distance_to_scrollbar,
-                 DeviceSpaceDistanceToLayer(device_viewport_point, scrollbar));
-  new_animation_controller->DidMouseMoveNear(
-      distance_to_scrollbar / active_tree_->device_scale_factor());
+  for (ScrollbarLayerImplBase* scrollbar : ScrollbarsFor(new_id)) {
+    new_animation_controller->DidMouseMoveNear(
+        scrollbar->orientation(),
+        DeviceSpaceDistanceToLayer(device_viewport_point, scrollbar) /
+            active_tree_->device_scale_factor());
+  }
+  new_animation_controller->EnsureScrollbarFadeIn();

[bokan, 2016/12/16 14:57:42]

Make this private and move it into DidMouseMoveNear.
Also, shouldn't this only be called if the mouse is actually over the scrollbar?

[chaopeng, 2016/12/16 21:02:25]

No, we need to keep scrollbar fade in while the mouse is near. 

 }
 
 void LayerTreeHostImpl::MouseLeave() {
   for (auto& pair : scrollbar_animation_controllers_)
     pair.second->DidMouseLeave();
   scroll_layer_id_mouse_currently_over_ = Layer::INVALID_ID;
 }
 
 void LayerTreeHostImpl::PinchGestureBegin() {
   pinch_gesture_active_ = true;
@@ skipping 791 matching lines @@
       worker_context_visibility_ =
           worker_context->CacheController()->ClientBecameVisible();
     } else {
       worker_context->CacheController()->ClientBecameNotVisible(
           std::move(worker_context_visibility_));
     }
   }
 }
 
 }  // namespace cc