It isn't clear what other UI elements are affected beside the search box, if any.
I'm not sure what projects this applies to, since it is a UI problem on an element used for search.
Earlier discussion from T75605:
A bug, which may be related, still exists for Greek terms (in ALL projects, even in en.wiktionary) . Typing in the search box anything that ends in accented letter does not provide any suggestions that include the last letter (even if they exist ex. καλά). Copying and pasting works. Also typing anything after (ex. a space) works. It seems (to user) like the search is not done by the really typed letters, and the code is "waiting" for something.
For my quick test, I'm on a Mac, using the American, French, and Greek keyboards, and I tested in Chrome, Safari, and Firefox. To my surprise, they all behave the same.
As I understand it, the Greek keyboard uses a dead key to add ´ and ¨ to vowels. Similarly in the Mac American keyboard has dead keys for several diacritics (I use ´ ¨ ˆ ` ˜ regularly). If I type resumé to search on English Wiktionary, I also don't get any more suggestions for the final é. (BTW, it happens for non-final letters, too, if you pause, but it's easy to miss if you keep typing).
On the French keyboard, é has its own key, and when I type resumé using that keyboard, it behaves as expected.
On the Mac Greek keyboard (so this probably does not apply to Windows or Linux), I can type ά by typing option+shift+α. If I type καλά this way it gets suggestions as expected. Similarly, you can use option+shift+<x> to type other accented vowels: 1/έ 2/ί 3/ή 4/ό 0/ύ ./ώ —I didn't see any precomposed versions with diaeresis (i.e, ϊ or ϋ ). These non-dead-key versions generate new suggestions.
So, the problem isn't accented characters per se, but rather characters that have to be typed with dead keys, at least on a Mac keyboard. I'm not familiar with the UI code that's handling all this, so I have no idea how easy it would be to fix, but searching online shows a lot of people complaining about this, but no obvious solutions.
The problem is in the code for sure. The accent in dead keys in Greek keyboard are typed first so the last key pressed is a non dead key. onkeyup works in my tests.
(See link for attachments.)