Disable overlay scrollbars to hide them on non-Mac.

third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h

 /*
  * Copyright (C) 2008 Apple 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 53 matching lines @@
   virtual int scrollbarMargin() const { return 0; }
 
   virtual WebScrollbarButtonsPlacement buttonsPlacement() const {
     return WebScrollbarButtonsPlacementSingle;
   }
 
   virtual bool supportsControlTints() const { return false; }
   virtual bool usesOverlayScrollbars() const { return false; }
   virtual void updateScrollbarOverlayColorTheme(const ScrollbarThemeClient&) {}
 
+  // If true, scrollbars that become invisible (i.e. overlay scrollbars that
+  // fade out) should be marked as disabled. This option exists since Mac and
+  // Aura overlays implement the fade out differently, with Mac painting code
+  // fading out the scrollbars. Aura scrollbars require disabling the scrollbar
+  // to prevent painting it.
+  virtual bool disableInvisibleScrollbars() const { return true; }

[jbroman, 2016/11/18 19:00:27]

nit: this name suggests that calling it disables the scrollbars. Could it be
named "disablesInvisibleScrollbars()" (like usesOverlayScrollbars above), or
"shouldDisableInvisibleScrollbars()", or something along those lines?

[bokan, 2016/11/18 19:09:00]

Good point, done.

+
   virtual bool invalidateOnMouseEnterExit() { return false; }
   virtual bool invalidateOnWindowActiveChange() const { return false; }
 
   // Returns parts of the scrollbar which must be repainted following a change
   // in the thumb position, given scroll positions before and after.
   virtual ScrollbarPart invalidateOnThumbPositionChange(
       const ScrollbarThemeClient&,
       float oldPosition,
       float newPosition) const {
     return AllParts;
@@ skipping 97 matching lines @@
   static DisplayItem::Type trackPiecePartToDisplayItemType(ScrollbarPart);
 
  private:
   static ScrollbarTheme&
   nativeTheme();  // Must be implemented to return the correct theme subclass.
   static bool gMockScrollbarsEnabled;
 };
 
 }  // namespace blink
 #endif