Call ScrollableArea::ShowOverlayScrollbars for explicit scrolls only.

third_party/WebKit/Source/core/frame/VisualViewport.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 619 matching lines @@
   }
   return rect;
 }
 
 RefPtr<WebTaskRunner> VisualViewport::GetTimerTaskRunner() const {
   return TaskRunnerHelper::Get(TaskType::kUnspecedTimer, MainFrame());
 }
 
 void VisualViewport::UpdateScrollOffset(const ScrollOffset& position,
                                         ScrollType scroll_type) {
-  if (DidSetScaleOrLocation(scale_, FloatPoint(position)) &&
-      scroll_type != kAnchoringScroll)
+  if (!DidSetScaleOrLocation(scale_, FloatPoint(position)))
+    return;
+  if (IsExplicitScrollType(scroll_type)) {

[pdr., 2017/04/26 00:28:32]

This will no longer call NotifyRootFrameViewport when the scroll type is
kClampingScroll. Is that intentional?

[skobes, 2017/04/26 01:28:57]

Yes, I think it was a mistake to call it for kClampingScroll. 
NotifyRootFrameViewport exists to call ScrollAnchor::Clear which should only
occur after explicit scrolls.

     NotifyRootFrameViewport();
+    if (scroll_type != kCompositorScroll && LayerForScrolling())
+      LayerForScrolling()->PlatformLayer()->ShowScrollbars();
+  }
 }
 
 GraphicsLayer* VisualViewport::LayerForContainer() const {
   return inner_viewport_container_layer_.get();
 }
 
 GraphicsLayer* VisualViewport::LayerForScrolling() const {
   return inner_viewport_scroll_layer_.get();
 }
 
@@ skipping 199 matching lines @@
   } else if (graphics_layer == root_transform_layer_.get()) {
     name = "Root Transform Layer";
   } else {
     ASSERT_NOT_REACHED();
   }
 
   return name;
 }
 
 }  // namespace blink