User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0

Steps to reproduce:

I'm actually not sure if this is a bug in the display server, the window manager, the GPU driver, or Firefox itself, but since it does happen on Firefox, I thought I'd report it here, feel free to close or forward it someplace else if you deem it's more appropriate.

In any case, what I did was set gfx.webender.all to true in ESR 68.6.0esr-1, running on Debian, XFCE 4.14 w/xfwm4 on an Intel UHD620.

Actual results:

When the Firefox window is not in focus, and I switch from one workspace to another and then back, the FF window is completely blank on a fresh user (see attachment), or shows graphical glitches on the current one (not sure if it's my about:config setup or some extension that's responsible).
Curiously, if I move the mouse to the blanked window, and leave it inside for about a second, it shows the content, and it actually displays the content instantly if I hover over any of the interactive elements (the X to close a tab, search bar, back button etc.).

Expected results:

It should display the current content instead of being blank. Note that this only happens when WebRender is enabled, no other setting has been changed.

Bugbug thinks this bug should belong to this component, but please revert this change in case of error.

Could you try the nightly (with WR enabled)?
Also, please attach the "about:support" output.

Any immediate ideas, Andrew? Looks like we aren't receiving the window refresh() event when not in focus on X11.

(In reply to Dzmitry Malyshau [:kvark] from comment #2)

Could you try the nightly (with WR enabled)?
Also, please attach the "about:support" output.

Attached about:support in comment above.
The bug does not seem to be present in Nightly with Webrender enabled, more specifically v76.0a1.

Alright, great, thank you!
So this is now about whether we need to find the exact fix and back-port it to the ESR.
This step requires bisecting with mozregression to find the exact point it got fixed.

(In reply to jcgoran)

In any case, what I did was set gfx.webender.all to true in ESR 68.6.0esr-1, running on Debian, XFCE 4.14 w/xfwm4 on an Intel UHD620.

The bug does not seem to be present in Nightly with Webrender enabled, more specifically v76.0a1.

bug 1571331 fixed this for KDE (with disabled compositor) in Firefox 70. I assume this might have fixed it for you, too.
bug 1611372 was another fix in Firefox 77. Better use Firefox Stable to get such fixes faster.

(Dzmitry Malyshau [:kvark] from comment #6)

This step requires bisecting with mozregression to find the exact point it got fixed.

The commands would have been:

$ pip install --upgrade mozregression
$ mozregression --find-fix --bad 2019-05-20 --good 2020-04-06 --pref gfx.webrender.all:true