RTL overlay scrollbar on Mac is left aligned.

third_party/WebKit/Source/platform/scroll/ScrollbarThemeMac.mm

 /*
  * Copyright (C) 2008, 2011 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 275 matching lines @@
   DrawingRecorder recorder(context, scrollbar, DisplayItem::kScrollbarThumb,
                            dirtyRect);
 
   GraphicsContextStateSaver stateSaver(context);
   context.translate(rect.x(), rect.y());
   LocalCurrentGraphicsContext localContext(context,
                                            IntRect(IntPoint(), rect.size()));
 
   ScrollbarPainter scrollbarPainter = painterForScrollbar(scrollbar);
   [scrollbarPainter setEnabled:scrollbar.enabled()];
-  [scrollbarPainter setBoundsSize:NSSizeFromCGSize(rect.size())];
+  // drawKnob aligns the thumb to right side of the draw rect.
+  // If the vertical overlay scrollbar is on the left, use trackWidth instead
+  // of scrollbar width, to avoid the gap on the left side of the thumb.

[jbroman, 2016/11/22 23:21:54]

One question here: what happens if the Mac system language is set to an RTL
language?

It'd be nice to be sure that |drawKnob| actually draws on the right, as opposed
to "right on LTR, left on RTL" (in the latter case, there would also be the
reverse version of this bug, for LTR content on an RTL system).

[sahel, 2016/11/23 18:22:07]

I set the OS language to Farsi. The window scrollbars are still on the right.
This is also true for main frame scrollbars on Safari. Scrollbar of a div with
RTL direction doesn't go to the left either (regardless of the system language).

+  IntRect drawRect = IntRect(rect);
+  if (usesOverlayScrollbars() && scrollbar.isLeftSideVerticalScrollbar()) {
+    int thumbWidth = [scrollbarPainter trackWidth];
+    drawRect.setWidth(thumbWidth);
+  }
+  [scrollbarPainter setBoundsSize:NSSizeFromCGSize(drawRect.size())];
+
   [scrollbarPainter setDoubleValue:0];
   [scrollbarPainter setKnobProportion:1];
 
   CGFloat oldKnobAlpha = [scrollbarPainter knobAlpha];
   [scrollbarPainter setKnobAlpha:1];
 
   if (scrollbar.enabled())
     [scrollbarPainter drawKnob];
 
   // If this state is not set, then moving the cursor over the scrollbar area
@@ skipping 93 matching lines @@
 }
 
 // static
 NSScrollerStyle ScrollbarThemeMac::recommendedScrollerStyle() {
   if (RuntimeEnabledFeatures::overlayScrollbarsEnabled())
     return NSScrollerStyleOverlay;
   return gPreferredScrollerStyle;
 }
 
 }  // namespace blink