Milan Crha created an issue #483:
Something happened to the list of available labels when I want to assign some to the issue. I click "Edit", it opens the list and when I want to scroll the list with a mouse wheel (cursor above the list), then one step of the wheel scrolls much more, quite unexpectedly more; the same when I use the scrollbar buttons. It makes the list unusable without searching, but that means you know what labels are available.
I guess and only guess it's due to the long label names, which could be added only recently to the group of the common labels. That's a plain guess though. Maybe it's a regression after any recent update, if there was any (I see this for more than a week, I think, I only didn't dare to file it).
When I try it here, where all the label names are reasonably short (one-line), I cannot reproduce the misbehavior.
In case you'd like to upstream this, then, please, shorten the label names to workaround the bug until upstream fixes it (which can take ages).