Keep expanded if mouse moves off of scrollbar while dragging

cc/input/scrollbar_animation_controller_thinning.cc

Index: cc/input/scrollbar_animation_controller_thinning.cc
diff --git a/cc/input/scrollbar_animation_controller_thinning.cc b/cc/input/scrollbar_animation_controller_thinning.cc
index af02ad3cddb6d2ebf5b31d877bcecdba8abd8ac1..729323169262e9240487b00dee61d451e39b37d7 100644
--- a/cc/input/scrollbar_animation_controller_thinning.cc
+++ b/cc/input/scrollbar_animation_controller_thinning.cc
@@ -41,6 +41,7 @@ ScrollbarAnimationControllerThinning::ScrollbarAnimationControllerThinning(
                                    delay_before_starting,
                                    resize_delay_before_starting,
                                    duration),
+      captured_(false),
       mouse_is_over_scrollbar_(false),
       mouse_is_near_scrollbar_(false),
       thickness_change_(NONE),
@@ -65,15 +66,38 @@ void ScrollbarAnimationControllerThinning::RunAnimationFrame(float progress) {
   }
 }
 
+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);
@@ -87,21 +111,27 @@ void ScrollbarAnimationControllerThinning::DidMouseMoveNear(float distance) {
   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;
+    }
 
-  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();
   }
-
-  StartAnimation();
 }
 
 float ScrollbarAnimationControllerThinning::OpacityAtAnimationProgress(