Created attachment 8367477 [details] dropdownbug.gif See screencast. It seems to reproduce all the time right now. I'm on 29.0a1 (2014-01-28) + 10.9.
Hmm, I tried to find a regression range, but it turns out I'm unable to reproduce the issue. I'm on 10.9.1. Anything specific about your setup, profile or preferences that could be the trigger?
I'm starting to think my main profile is just messed up. I ran a regression window with my main profile and that's perhaps what has caused my firefox to become very buggy on OSX. This doesn't reproduce on a clean profile. I'll take a look and see if I can see what in my profiles is causing it.
I can also reproduce this without a clean profile on x86 Linux. I poked around in my prefs.js and what I found is that this happens with layers.offmainthreadcomposition.async-animations = true. If I flip it or disable omtc entirely, <select> works as expected.
4 years ago