Call ScrollableArea::ShowOverlayScrollbars for explicit scrolls only.

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights
  * reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@gmail.com>
@@ skipping 354 matching lines @@
   return (orientation == kHorizontalScrollbar) ? scroll_dimensions.Width()
                                                : scroll_dimensions.Height();
 }
 
 void PaintLayerScrollableArea::UpdateScrollOffset(
     const ScrollOffset& new_offset,
     ScrollType scroll_type) {
   if (GetScrollOffset() == new_offset)
     return;
 
-  ShowOverlayScrollbars();
   scroll_offset_ = new_offset;
 
   LocalFrame* frame = Box().GetFrame();
   DCHECK(frame);
 
   FrameView* frame_view = Box().GetFrameView();
 
   TRACE_EVENT1("devtools.timeline", "ScrollLayer", "data",
                InspectorScrollLayerEvent::Data(&Box()));
 
@@ skipping 79 matching lines @@
   Box().View()->ClearHitTestCache();
 
   // Inform the FrameLoader of the new scroll position, so it can be restored
   // when navigating back.
   if (Layer()->IsRootLayer()) {
     frame_view->GetFrame().Loader().SaveScrollState();
     frame_view->DidChangeScrollOffset();
   }
 
   if (IsExplicitScrollType(scroll_type)) {
+    if (scroll_type != kCompositorScroll)
+      ShowOverlayScrollbars();
     frame_view->ClearFragmentAnchor();
     if (RuntimeEnabledFeatures::scrollAnchoringEnabled())
       GetScrollAnchor()->Clear();
   }
 }
 
 IntSize PaintLayerScrollableArea::ScrollOffsetInt() const {
   return FlooredIntSize(scroll_offset_);
 }
 
@@ skipping 1637 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollOffsetClampScope::
     ClampScrollableAreas() {
   for (auto& scrollable_area : *needs_clamp_)
     scrollable_area->ClampScrollOffsetAfterOverflowChange();
   delete needs_clamp_;
   needs_clamp_ = nullptr;
 }
 
 }  // namespace blink