Layouts do not respect RTL

This is major (where "major" is "critical bug for one subsystem"). If we'd caught it before, it would have blocked making Layout Discovery stable.

This seems like a case I could help with, but I want to make sure I'm not misunderstanding the scale and scope of the issue.

For the layout_discovery module I see the following CSS files which use flexbox:

• threecol_25_50_25/threecol_25_50_25.css
• threecol_33_34_33/threecol_33_34_33.css
• twocol/twocol.css
• twocol_bricks/twocol_bricks.css

Are these the CSS files which need RTL support added? Are there any others?

Add explicit RTL support.

Am I correct that this should be done via adding [dir="rtl"] CSS selectors and changing the flex direction?

Flexbox takes the language direction into account on its own, so we do not need to add additional CSS to reverse the layouts for rtl languages.

This Mozilla documentation for Flexbox describes the behavior for row and row-reverse, but the same applies to wrap and wrap-reverse, which is what the layouts are using.