(In reply to Darkspirit from comment #8)
Well, it's all over the place. Glitches are unstable, so it's too easy to miss bisection turn and arrive to wrong conclusion. I've tried to track specifically glitches at right and bottom borders of About window. Since the bug title was changed to "GLX+EGL", I didn't force EGL:
./.local/bin/mozregression --pref gfx.webrender.all:true gfx.webrender.enabled:true browser.tabs.drawInTitlebar:true browser.tabs.inTitlebar:1 --good=2020-01-01
which gave me this: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=7c4202065cd61b0d45a5c82ab1c177752902b960&tochange=48913f408b556755f3195883a689e0b60ef356ed.
Maybe I'm missing something, but to me this changeset range doesn't have anything related to rendering.
Another time when I was chasing general rendering issues of the About window, like some transparent areas, or all-white areas, or all-black areas. It gave me this range: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=05dbdc65a0da4889b4675986e80d0ed9ddf5b821&tochange=5a65ca077fd746f5f977342f2e0fc112ed79ad31.
I've noticed once that I've got all-black About window with a Firefox version from 2019-01-01. Then I've tried to start mozregression at 2018-06-01, and it resulted in this range: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=9f9a9234959f114825f58beee0cffbab82d0bb29&tochange=6e842238034cd847ede178b4e65ea07704e4ffe6. During this run I opened About window dozens of times. Rendering issues (all-white or all-black windows) are hard to reproduce, so this range is likely to be wrong.
And it seems that
browser.tabs.drawInTitlebar:true is not strictly related to glitches, as I've seen all-black window with system title bar visible.