Neither vertical-tabs-rtl.css from core/misc or from Seven are recognized for RTL. Vertical tabs are not a module, its 2 stylesheets and 1 js file are at the top level of core/misc. The only other stylesheets at that location are print. The 2 stylesheets are overridden in Seven.

Dev tools show no vertical-tabs-rtl.css used while displaying RTL.
Screenshot is showing color red tested in Seven's vertical-tabs.css but not showing an override of color green tested in Seven's vertical-tabs-rtl.css (in addition to not rendering any of the RTL css).

no-rtl-css-vertical-tabs.pngtest-vertical-tabs.pngtest-vertical-tabs-rtl.png

Comments

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An oddity that is perhaps a clue, seven's vertical-tabs.css is called twice in the RTL source. Not so for any other stylesheet.

source-rtl.png

Status:Active» Fixed

Resolved with #2015789: Remove language_css_alter() (RTL stylesheets) in favor of HTML 'dir' attribute !
Marking as fixed because I'm confident 2015789 is going through :-)

Automatically closed -- issue fixed for 2 weeks with no activity.