User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:74.0) Gecko/20100101 Firefox/74.0

Steps to reproduce:

Nightly Build used When Posted: 74.0a1 (2020-01-22) (64-bit)
Have Extended monitors:

Open 2 instances / windows of Firefox Nightly.
Put 1 window full screen on each of the displays (idk if this matters but i use 720p res on both).
Open Youtube on 1 and play any video and full screen video.
While the other instance is maximized scroll around on pages / type.

Actual results:

Full Screen video will continuously flicker black for the whole duration your either typing or scrolling on a page on the 2nd maximized window.

Expected results:

Nothing.

Hi,
thanks for submitting this bug to us!

I was unable to reproduce this issue on my end, I tried on Windows 10 with Firefox Nightly 74.0a1 (2020-01-27) and Release 72.0.2 (64-bit) versions.

Does this issue occur with a fresh profile as well? You can find the steps here: https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles?redirectlocale=en-US&redirectslug=Managing-profiles#w_starting-the-profile-manager

You can also try testing if the issue is reproducible in safe mode, here is a link that can help you:
https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode

If after doing this you can still reproduce the bug, would you send a screenshot or a video of the bug?

I've already chosen a component for this bug in hope that someone with more expertise may look at it. We'll await their answer.

Regards,
Jerónimo.

Not sure if syncing had anything to do with it because i went from the normal build of Firefox to the Nightly Build of it and used my account to sync all settings, password etc... to the Nightly Version, Sorry to waste any of your time, however after the latest nightly updates i can no longer reproduce it either, it used to happen any time i had two Firefox nightly windows open on full screen and normally browsed the web on one while watching a YouTube video on the other one on my TV, but now it no longer does it anymore, i didn't have to do anything to resolve this the updates just eventually stopped it after the latest few, no clue whether or not it will return or reoccur in later versions but currently it no longer happens.

It looks similar to Bug 1586929.

If the problem does not happen with latest nightly, Bug 1592509 might address the problem. It enables DirectComposition layer usage and disables SwapChain usage.