Is this possible? Would need us to build the select widget in styled HTML, which… isn't possible for non-JS clients or something?

It's almost impossible to provide a dropdown looking identically to the one we generate with JavaScript, that also actually works, without JavaScript. (We haven't quite rolled out the heavy CSS artillery here, but it would require a lot of fiddling.)

Although, I looked carefully took some screenshots and this looks a bit worse than I thought it does: the height of the dropdown actually changes with JS, shifting the rest of the page down, which is definitely not supposed to happen (and I think it wasn't happening the last time I tested this).

Before flash	After flash

I'll look into ensuring that the no-JS and JS dropdowns have the exact same dimensions.

Change 242530 had a related patch set uploaded (by Bartosz Dziewoński):
MediaWiki theme: Make PHP DropdownInputWidget look closer to JS version

https://gerrit.wikimedia.org/r/242530

The flash should become nearly imperceptible with the patch above. (How imperceptible depends on how close your browser's and OS's dropdowns are to OOjs UI's, but the dimensions at least should stay the same.)

Change 242530 merged by jenkins-bot:
MediaWiki theme: Make PHP DropdownInputWidget look closer to JS version

https://gerrit.wikimedia.org/r/242530