Overlay scrollbars flush with window edge

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

Index: third_party/WebKit/Source/platform/scroll/ScrollbarThemeOverlay.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollbarThemeOverlay.cpp b/third_party/WebKit/Source/platform/scroll/ScrollbarThemeOverlay.cpp
index 30ba2b9554699d95115ee07d6d3ed8f1506b2dd7..b7a3838cf7313d5f5c382bb9fb4857971bdff0cc 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollbarThemeOverlay.cpp
+++ b/third_party/WebKit/Source/platform/scroll/ScrollbarThemeOverlay.cpp
@@ -205,8 +205,18 @@ void ScrollbarThemeOverlay::paintThumb(GraphicsContext& context,
       static_cast<WebScrollbarOverlayColorTheme>(
           scrollbar.getScrollbarOverlayColorTheme());
 
+  // Horizontally flip the canvas if it is left vertical scrollbar.

[jbroman, 2017/03/29 20:51:35]

Is the scrollbar guaranteed to be horizontally symmetrical? Not everything
should be visually flipped for RTL. (e.g. if we drew shadows, they would seem to
look wrong with this 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(