Overlay scrollbars flush with window edge

third_party/WebKit/Source/platform/scroll/ScrollbarThemeOverlay.cpp

 /*
  * Copyright (C) 2011 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:
  *
  * 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
@@ skipping 187 matching lines @@
 
   WebThemeEngine::Part part = WebThemeEngine::PartScrollbarHorizontalThumb;
   if (scrollbar.orientation() == VerticalScrollbar)
     part = WebThemeEngine::PartScrollbarVerticalThumb;
 
   blink::WebThemeEngine::ExtraParams params;
   params.scrollbarThumb.scrollbarTheme =
       static_cast<WebScrollbarOverlayColorTheme>(
           scrollbar.getScrollbarOverlayColorTheme());
 
+  // Filp the canvas if it is left vertical scrollbar.

[Evan Stade, 2017/03/29 18:05:42]

nit: s/Filp/Horizontally flip/

+  if (scrollbar.isLeftSideVerticalScrollbar()) {
+    canvas->save();
+    canvas->translate(canvas->getBaseLayerSize().width(), 0);
+    canvas->scale(-1, 1);
+  }
+
   Platform::current()->themeEngine()->paint(canvas, part, state, WebRect(rect),
                                             &params);
+
+  if (scrollbar.isLeftSideVerticalScrollbar())
+    canvas->restore();
 }
 
 ScrollbarPart ScrollbarThemeOverlay::hitTest(
     const ScrollbarThemeClient& scrollbar,
     const IntPoint& position) {
   if (m_allowHitTest == DisallowHitTest)
     return NoPart;
 
   ScrollbarPart part = ScrollbarTheme::hitTest(scrollbar, position);
   if (part != ThumbPart)
@@ skipping 44 matching lines @@
 
   WebThemeEngine::Part part = WebThemeEngine::PartScrollbarHorizontalThumb;
   if (scrollbar.orientation() == VerticalScrollbar)
     part = WebThemeEngine::PartScrollbarVerticalThumb;
 
   DCHECK(Platform::current()->themeEngine());
   return Platform::current()->themeEngine()->ninePatchAperture(part);
 }
 
 }  // namespace blink