Hi msw@, there is a related, possible bug in RTL: the text direction. 

Because there could be many child-labels in one line of StyledLabel, it is challenge to read the whole text after dividing the text into multiple labels in RTL but the text direction is not RTL. Is this possible? 

Please see the attached image. Should I file a new bug or there is one already?

I don't think this is a really bad problem; it's the nature of doing RTL layout with LTR text.
It's definitely unrelated to this bug about mojo app content not respecting the UI direction.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1d02badfaa943d7f65d5f58998392045f697df36

commit 1d02badfaa943d7f65d5f58998392045f697df36
Author: Mike Wasserman <msw@chromium.org>
Date: Wed Jun 27 19:52:24 2018

Propagate forced direction switches to the utility process

Views mojo apps (eg. Keyboard Shortcut Viewer) will use these switches.

Bug:  856341 
Test: cros KSV (Ctrl-Alt-/) respects --force-[ui|text]-direction=rtl
Change-Id: I7f0a2577c6c8dab362125f385305521cd624705f
Reviewed-on: https://chromium-review.googlesource.com/1116003
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570871}
[modify] https://crrev.com/1d02badfaa943d7f65d5f58998392045f697df36/content/browser/utility_process_host.cc