Similar to 860627 but unlike that one, this is a supported use case (assuming you did restart?)

I think making this return false if 10.11 or less and RTL would do it, I'll check next time I have a 10.11 VM handy.
https://cs.chromium.org/chromium/src/chrome/browser/ui/views/frame/browser_non_client_frame_view_mac.mm?type=cs&q=CaptionButtonsOnLeadingEdge&sq=package:chromium&g=0&l=74

What do you mean, "assuming you did restart"?

I'm happy to do it; what's the best way to check RTL in that file?

Re c#4, there's this weird quirk where just changing languages doesn't fully prod the system into RTL; instead you need to log in and out or restart. You can get around this with 
-NSForceRightToLeftWritingDirection YES -AppleTextDirection YES

Re c#5, base::i18n::IsRTL should do it. Thanks for trying! 

OK, though AFAIK in 10.11 and before the traffic lights never move.

Alas, just

bool BrowserNonClientFrameViewMac::CaptionButtonsOnLeadingEdge() const {
  // On 10.11 and earlier, the caption buttons are always on the left edge, even
  // in RTL.
  if (base::i18n::IsRTL() && base::mac::IsAtMostOS10_11())
    return false;

  return true;
}

doesn't work.

I'll let you have a try.

** Mass UI Triage **

We were unable to reproduce this bug on Mac 10.13.6 with chrome #72.0.3610.0. If this bug still reproduces for you, please reopen or file a new issue. Thanks!

It's a 10.11 bug, so it's expected that it won't reproduce in 10.13.6

Reopening (but unassigning since I won't get to it soon)