Keep expanded if mouse moves off of scrollbar while dragging

cc/input/scrollbar_animation_controller_thinning.cc

 // Copyright 2013 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/input/scrollbar_animation_controller_thinning.h"
 
 #include "base/memory/ptr_util.h"
 #include "base/time/time.h"
 #include "cc/layers/layer_impl.h"
 #include "cc/layers/scrollbar_layer_impl_base.h"
@@ skipping 23 matching lines @@
     int scroll_layer_id,
     ScrollbarAnimationControllerClient* client,
     base::TimeDelta delay_before_starting,
     base::TimeDelta resize_delay_before_starting,
     base::TimeDelta duration)
     : ScrollbarAnimationController(scroll_layer_id,
                                    client,
                                    delay_before_starting,
                                    resize_delay_before_starting,
                                    duration),
+      captured_(false),
       mouse_is_over_scrollbar_(false),
       mouse_is_near_scrollbar_(false),
       thickness_change_(NONE),
       opacity_change_(NONE),
       mouse_move_distance_to_trigger_animation_(
           kDefaultMouseMoveDistanceToTriggerAnimation) {
   ApplyOpacityAndThumbThicknessScale(kIdleOpacity, kIdleThicknessScale);
 }
 
 ScrollbarAnimationControllerThinning::~ScrollbarAnimationControllerThinning() {}
 
 void ScrollbarAnimationControllerThinning::RunAnimationFrame(float progress) {
   float opacity = OpacityAtAnimationProgress(progress);
   float thumb_thickness_scale =
       ThumbThicknessScaleAtAnimationProgress(progress);
   ApplyOpacityAndThumbThicknessScale(opacity, thumb_thickness_scale);
   client_->SetNeedsRedrawForScrollbarAnimation();
   if (progress == 1.f) {
     opacity_change_ = NONE;
     thickness_change_ = NONE;
     StopAnimation();
   }
 }
 
+void ScrollbarAnimationControllerThinning::DidCaptureScrollbarBegin() {
+  captured_ = true;
+  ApplyOpacityAndThumbThicknessScale(1, 1.f);

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

I see that 1.f is the maximum opacity in OpacityAtAnimationProgress but when I
check, the scrollbars are never fully opaque, they're still a little see through
even when the mouse is over them. Do you know why?

[chaopeng, 2016/09/23 02:38:26]

I log the opacity in OpacityAtAnimationProgress and
effect_tree.OnOpacityAnimated. It is 1.f. 

But here looks like the method to draw. 
https://cs.chromium.org/chromium/src/cc/layers/solid_color_scrollbar_layer_im...

the color is 128, 128, 128, 128
https://cs.chromium.org/chromium/src/blimp/client/support/compositor/blimp_la...

+}
+
+void ScrollbarAnimationControllerThinning::DidCaptureScrollbarEnd() {
+  captured_ = false;
+
+  if (!mouse_is_over_scrollbar_)
+    opacity_change_ = DECREASE;
+  if (!mouse_is_near_scrollbar_)
+    thickness_change_ = DECREASE;
+  StartAnimation();
+}
+
 void ScrollbarAnimationControllerThinning::DidMouseMoveOffScrollbar() {
   mouse_is_over_scrollbar_ = false;
   mouse_is_near_scrollbar_ = false;
+
+  if (captured_)
+    return;
+
   opacity_change_ = DECREASE;
   thickness_change_ = DECREASE;
   StartAnimation();
 }
 
 void ScrollbarAnimationControllerThinning::DidScrollUpdate(bool on_resize) {
+  if (captured_) {
+    return;
+  }
+
   ScrollbarAnimationController::DidScrollUpdate(on_resize);
   ApplyOpacityAndThumbThicknessScale(
       1, mouse_is_near_scrollbar_ ? 1.f : kIdleThicknessScale);
 
   if (!mouse_is_over_scrollbar_)
     opacity_change_ = DECREASE;
 }
 
 void ScrollbarAnimationControllerThinning::DidMouseMoveNear(float distance) {
   bool mouse_is_over_scrollbar = distance == 0.0;
   bool mouse_is_near_scrollbar =
       distance < mouse_move_distance_to_trigger_animation_;
 
-  if (mouse_is_over_scrollbar == mouse_is_over_scrollbar_ &&
-      mouse_is_near_scrollbar == mouse_is_near_scrollbar_)
+  if (captured_) {
+    mouse_is_near_scrollbar_ = mouse_is_near_scrollbar;
+    mouse_is_over_scrollbar_ = mouse_is_over_scrollbar;
     return;
+  } else {
+    if (mouse_is_over_scrollbar == mouse_is_over_scrollbar_ &&
+        mouse_is_near_scrollbar == mouse_is_near_scrollbar_)
+      return;
 
-  if (mouse_is_over_scrollbar_ != mouse_is_over_scrollbar) {
-    mouse_is_over_scrollbar_ = mouse_is_over_scrollbar;
-    opacity_change_ = mouse_is_over_scrollbar_ ? INCREASE : DECREASE;
+    if (mouse_is_over_scrollbar_ != mouse_is_over_scrollbar) {
+      mouse_is_over_scrollbar_ = mouse_is_over_scrollbar;
+      opacity_change_ = mouse_is_over_scrollbar_ ? INCREASE : DECREASE;
+    }
+
+    if (mouse_is_near_scrollbar_ != mouse_is_near_scrollbar) {
+      mouse_is_near_scrollbar_ = mouse_is_near_scrollbar;
+      thickness_change_ = mouse_is_near_scrollbar_ ? INCREASE : DECREASE;
+    }
+
+    StartAnimation();
   }
-
-  if (mouse_is_near_scrollbar_ != mouse_is_near_scrollbar) {
-    mouse_is_near_scrollbar_ = mouse_is_near_scrollbar;
-    thickness_change_ = mouse_is_near_scrollbar_ ? INCREASE : DECREASE;
-  }
-
-  StartAnimation();
 }
 
 float ScrollbarAnimationControllerThinning::OpacityAtAnimationProgress(
     float progress) {
   if (opacity_change_ == NONE)
     return mouse_is_over_scrollbar_ ? 1.f : kIdleOpacity;
   float factor = opacity_change_ == INCREASE ? progress : (1.f - progress);
   float ret = ((1.f - kIdleOpacity) * factor) + kIdleOpacity;
   return ret;
 }
@@ skipping 42 matching lines @@
           property_trees->effect_id_to_index_map[scrollbar->id()],
           scrollbar->layer_tree_impl());
     }
     scrollbar->SetThumbThicknessScaleFactor(AdjustScale(
         thumb_thickness_scale, scrollbar->thumb_thickness_scale_factor(),
         thickness_change_));
   }
 }
 
 }  // namespace cc