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 4906ef4737db1921018b018ad1b8243652714805..da3ba8a2e204235b935a5fa88d0dc97c5c5ff269 100644
--- a/cc/input/scrollbar_animation_controller.cc
+++ b/cc/input/scrollbar_animation_controller.cc
@@ -53,6 +53,7 @@ ScrollbarAnimationController::ScrollbarAnimationController(
       scroll_gesture_has_scrolled_(false),
       opacity_(0.0f),
       fade_out_duration_(fade_out_duration),
+      show_scrollbars_on_scroll_gesture_(false),
       need_thinning_animation_(false),
       weak_factory_(this) {
   ApplyOpacityToScrollbars(0.0f);
@@ -77,6 +78,7 @@ ScrollbarAnimationController::ScrollbarAnimationController(
       scroll_gesture_has_scrolled_(false),
       opacity_(0.0f),
       fade_out_duration_(fade_out_duration),
+      show_scrollbars_on_scroll_gesture_(true),
       need_thinning_animation_(true),
       weak_factory_(this) {
   vertical_controller_ = SingleScrollbarAnimationControllerThinning::Create(
@@ -166,10 +168,6 @@ 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();
@@ -177,7 +175,26 @@ 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_ && MouseIsNearAnyScrollbar())
+    return;
+
+  if (has_scrolled)
+    PostDelayedFadeOut(false);
+}
+
+void ScrollbarAnimationController::DidScrollUpdate() {
   if (need_thinning_animation_ && Captured())
     return;
 
@@ -189,7 +206,7 @@ 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_ || !MouseIsNearAnyScrollbar())
-      PostDelayedFadeOut(on_resize);
+      PostDelayedFadeOut(false);
   } else {
     scroll_gesture_has_scrolled_ = true;
   }
@@ -202,19 +219,17 @@ 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_ && MouseIsNearAnyScrollbar())
-    return;
+void ScrollbarAnimationController::WillUpdateScroll() {
+  if (show_scrollbars_on_scroll_gesture_)
+    DidScrollUpdate();
+}
 
-  if (has_scrolled)
-    PostDelayedFadeOut(false);
+void ScrollbarAnimationController::DidResize() {
+  StopAnimation();
+  Show();
+  // 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() {