User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:82.0) Gecko/20100101 Firefox/82.0

Steps to reproduce:

1. enable EGL via MOZ_X11_EGL=1
2. click on the hamburger icon, or any of the extension icons to bring up a popup.

Actual results:

There is black borders around those popups.

Expected results:

Those part should be transparent, nice-looking.

I'm on Linux and X11 and I've checked that my compositor works. This is a regression, and mozregression finds:

5:02.73 INFO: No more integration revisions, bisection finished. 5:02.73 INFO: Last good revision: 645d922a048ce0d9ef0a3ba6d50ce9f05011f80a
5:02.73 INFO: First bad revision: e9d534d12e77469312104876f513cb749c25cce5
5:02.73 INFO: Pushlog:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=645d922a048ce0d9ef0a3ba6d50ce9f05011f80a&tochange=e9d534d12e774693121
04876f513cb749c25cce5

Seems duplication of Bug 1663003