User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:85.0) Gecko/20100101 Firefox/85.0

Steps to reproduce:

Load a youtube 4k video ( same problem occurs in vimeo).
The exact same video I loaded yesterday and was rendering fine, breaks with today's nightly on Linux (2020-11-21)
For example: https://www.youtube.com/watch?v=8HZ4DnVfWYQ&t=6s

Actual results:

The video playback breaks.
Firefox can only playback properly in 720p but anything higher than that breaks the rendering.
In vimeo it has to be 360p, even 720p breaks it.
Tried it in both my normal profile and a new one.

Expected results:

Until the previous nightly build all videos were playing flawlessly. I guess there was some regression with today's build.

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

Thanks for your report. Could you try using mozregression to check for a regression range? This would help a lot in narrowing down what change introduced the bug. I'd recommend the gui version of the tool for ease of use, and there is also a command line version.

Here you are:

12:13.12 INFO: Last good revision: 81361995e7ed2ba370867bfbefcba44f09732d22
12:13.12 INFO: First bad revision: 4bc0bdcd7889aab2994b4500d41e6e35216e4848
12:13.12 INFO: Pushlog:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=81361995e7ed2ba370867bfbefcba44f09732d22&tochange=4bc0bdcd7889aab2994b4500d41e6e35216e4848

I've never used mozregression before. It gave a few warnings for skipped builds, both good and bad. To be on the safe side I 'll include them as well ->

Bad:
6:52.25 WARNING: Skipping build 31395a8cf477: Unable to find build info using the taskcluster route 'gecko.v2.mozilla-central.shippable.revision.31395a8cf477aa4c52e2edf226c363e990dce8ec.firefox.linux64-opt'
9:40.30 WARNING: Skipping build 5153f0eaf518: Unable to find build info using the taskcluster route 'gecko.v2.autoland.shippable.revision.5153f0eaf51828659fb99fbe9039cd9ae1f79421.firefox.linux64-opt'
9:41.34 WARNING: Skipping build 345cccc10679: Unable to find build info using the taskcluster route 'gecko.v2.autoland.shippable.revision.345cccc106798a15626af4fd45e265db3adf9bac.firefox.linux64-opt'
9:42.41 WARNING: Skipping build f9537fedd47d: Unable to find build info using the taskcluster route 'gecko.v2.autoland.shippable.revision.f9537fedd47d8dbb27dfb19446a9d0de642edae4.firefox.linux64-opt'
10:29.47 WARNING: Skipping build d57553438d94: Unable to find build info using the taskcluster route 'gecko.v2.autoland.shippable.revision.d57553438d94c340a3c617449aab27665bd665fa.firefox.linux64-opt'
10:30.57 WARNING: Skipping build 5bf85f5f74c1: Unable to find build info using the taskcluster route 'gecko.v2.autoland.shippable.revision.5bf85f5f74c1262d2dad3283b111b2e388e9c2ca.firefox.linux64-opt'

Good:
11:14.03 WARNING: Skipping build fded65adecd4: Unable to find build info using the taskcluster route 'gecko.v2.autoland.shippable.revision.fded65adecd4639ac0f1db0cf11d6092c9e55a51.firefox.linux64-opt'

It looks like there's a webrender change in those patches which I imagine could lead to performance changes. Could you try navigating to about:config, setting the gfx.webrender.force-disabled pref to true, and restarting Firefox and seeing if that has any affect?

Switching gfx.webrender.force-disabled to true does resolve the issue.
What I forgot to mention earlier is that the problem occurs only with the NVIDIA drivers. I have another machine with NVIDIA graphics running the nouveau drivers and it runs fine there.
I guess you are referring to bug 1677293 which is closed. Would it make any sense if I comment there?

Thank you for your work!

Thanks for checking that. I'll move this to gfx based on the above information, and they should be able to help further. No need to comment on that bug, I'll update the information on each to indicate the relationship between the bugs.

sonickydon, can you get a profile using https://profiler.firefox.com/ with "Firefox Graphics" preset?

I hope this helps:
https://share.firefox.dev/2Vub9wg

sonickydon, can you describe in more detail what the "breaks the rendering" looks like?

Were you playing a single video for the entire duration of that profile (80s worth)? Do you have other windows open?

I had only one tab open and yes, I left the video play for a minute or so to be sure you'll get more info.

I'll try to describe it but english is not my mother tongue so bear with me.
It's not consistent, usually at first it looks like the video pane is split and while 1/3 of it is playing, 2/3 freeze on a random frame. Some times it turns to a small pane in the upper left corner which keeps playing while the rest is frozen and then it switches back to the previous state.

I've uploaded a couple of screenshots on google drive:
https://drive.google.com/drive/folders/1qycEW6_4oPchfR99u9gWdxEktAvnV743?usp=sharing

Ok, that is indeed very broken.

Correction: It looks like it switches from the "upper left corner pane" mode to 1/3 vertical if there's mouse movement. If i let it play without touching anything it even tries to playback properly, yet it's obvious that there's an upper left corner pane which displays a frame or two ahead of the rest of the screen.

Most videos have been broken for 3 weeks. This should be fixed now.
Please set gfx.webrender.force-disabled back to false. Thanks! :)