Show Overlay Scrollbar when GestureScrollUpdate

cc/input/scrollbar_animation_controller.cc

Index: cc/input/scrollbar_animation_controller.cc
diff --git a/cc/input/scrollbar_animation_controller.cc b/cc/input/scrollbar_animation_controller.cc
index 6927d0415738fc93435406bdeb0fcd3f8f30db1d..537b6cc22a9843f2c474828925c4512cdae5f836 100644
--- a/cc/input/scrollbar_animation_controller.cc
+++ b/cc/input/scrollbar_animation_controller.cc
@@ -162,10 +162,6 @@ float ScrollbarAnimationController::AnimationProgressAtTime(
   return std::max(std::min(progress, 1.f), 0.f);
 }
 
-void ScrollbarAnimationController::DidScrollBegin() {
-  currently_scrolling_ = true;
-}
-
 void ScrollbarAnimationController::RunAnimationFrame(float progress) {
   ApplyOpacityToScrollbars(1.f - progress);
   client_->SetNeedsRedrawForScrollbarAnimation();
@@ -173,7 +169,26 @@ void ScrollbarAnimationController::RunAnimationFrame(float progress) {
     StopAnimation();
 }
 
-void ScrollbarAnimationController::DidScrollUpdate(bool on_resize) {
+void ScrollbarAnimationController::DidScrollBegin() {
+  currently_scrolling_ = true;
+}
+
+void ScrollbarAnimationController::DidScrollEnd() {
+  bool has_scrolled = scroll_gesture_has_scrolled_;
+  scroll_gesture_has_scrolled_ = false;
+
+  currently_scrolling_ = false;
+
+  // We don't fade out scrollbar if they need thinning animation and mouse is
+  // near.
+  if (need_thinning_animation_ && mouse_is_near_any_scrollbar())
+    return;
+
+  if (has_scrolled)
+    PostDelayedFadeOut(false);
+}
+
+void ScrollbarAnimationController::WillUpdateScroll() {
   if (need_thinning_animation_ && Captured())
     return;
 
@@ -185,7 +200,7 @@ void ScrollbarAnimationController::DidScrollUpdate(bool on_resize) {
     // We don't fade out scrollbar if they need thinning animation and mouse is
     // near.
     if (!need_thinning_animation_ || !mouse_is_near_any_scrollbar())
-      PostDelayedFadeOut(on_resize);
+      PostDelayedFadeOut(false);
   } else {
     scroll_gesture_has_scrolled_ = true;
   }
@@ -198,19 +213,12 @@ void ScrollbarAnimationController::DidScrollUpdate(bool on_resize) {
   }
 }
 
-void ScrollbarAnimationController::DidScrollEnd() {
-  bool has_scrolled = scroll_gesture_has_scrolled_;
-  scroll_gesture_has_scrolled_ = false;
-
-  currently_scrolling_ = false;
-
-  // We don't fade out scrollbar if they need thinning animation and mouse is
-  // near.
-  if (need_thinning_animation_ && mouse_is_near_any_scrollbar())
-    return;
-
-  if (has_scrolled)
-    PostDelayedFadeOut(false);
+void ScrollbarAnimationController::DidResize() {
+  StopAnimation();
+  FadeIn();
+  // We should use the gesture delay rather than the resize delay if we're in a
+  // gesture scroll, even if it is resizing.
+  PostDelayedFadeOut(!currently_scrolling_);
 }
 
 void ScrollbarAnimationController::DidMouseDown() {