If you have configured Windows to enable both a RTL and a LTR keyboard layout (e.g. English and Persian), then you should be able to press CTRL+SHIFT to change the directionality of a textarea or textbox from RTL to LTR and back. This is a very handy feature we use on fawiki, because our wikitext may contain a mixture of text (which is RTL) and code (which would be LTR). Recently, this feature stopped working.
Originally, we suspected this may have to do with Desktop Improvements because it used to work until Desktop Improvements were turned on for fawiki. But it also has stopped working on fa.wikisource.org which still uses legacy Vector. Interestingly, it also doesn't work with Monobook anymore. Amazingly, it does work on Meta or en.wikipedia or en.wikisource
The fact that it doesn't work in both fa.wikipedia and fa.wikisource precludes it being caused by a Common.css rule (let alone the fact that I reviewed our Common.css changes and couldn't find a related change). Other RTL projects seem to also be affected (I tried he.wikipedia both logged in and logged out, and the keyboard shortcut did not work).
Also of note: the issue exists with Chrome and Firefox (have not tested other browsers), and both when logged in and logged out/incognito. This also precludes the possibility of this being caused by a browser upgrade (though I am going to install an older version of Firefox/Chrome and test with it too).