RTL overlay scrollbar on Mac is left aligned.

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

Index: third_party/WebKit/Source/platform/scroll/ScrollbarThemeMac.mm
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMac.mm b/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMac.mm
index 7b8c08cb22356b66ae2de29293c82575301d1fd0..88c92c377cb7d67cb53f5accb11b736f57d456b0 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMac.mm
+++ b/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMac.mm
@@ -293,7 +293,16 @@ void ScrollbarThemeMac::paintThumb(GraphicsContext& context,
 
   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];