I have no idea when it stopped working, because I scarcely ever used the affected buttons until very recently—I have a vague feeling it might have worked three or four years ago, but in the last couple of months it’s definitely not been working for me.
uBlock Origin is the only extension that I have that could feasibly alter anything, and all it’s doing is blocking googleads.g.doubleclick.net. Nothing of interest shows up in the dev tools. 99.9% sure the bug is with YouTube, though in what particular way I cannot guess.
Ah hah! Figured it out: some cosmetic filter is killing the dropdowns. Unfortunately the effecting rule is not discernable in the dev tools, so figuring out which rule is at fault is tricky and slow.
Found which one, too: it was a rule of my own that I added almost a year ago to kill off some of the annoying cards they pop up at the ends of many videos, which hide the video content; turns out my rule was just a touch too broad for how they do things.
