User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:76.0) Gecko/20100101 Firefox/76.0

Steps to reproduce:

This demo here:
https://codepen.io/j4k0bkg/pen/bGGpxjY

If you don't see any problem, try to resize the display frame of codepen.
In chrome or edge this does not happen.

Tested on Firefow Browser 76.0.1 (64bit) on Desktop PC

Actual results:

There are strange lines even going out of the browser-website-window up to the title bar (-> see demo)

Expected results:

no strange lines appearing

Hi,

I was able to reproduce the issue on Windows 10 using Firefox 76.0.1 and Firefox Nightly 78.0a1 with WebRender enabled.
If I set gfx.webrender.force-disabled to true (disable WebRender) the issue does not occur.

I'm setting a component in order to get a developer to take a look over this issue.

Thank you for reporting!

good: an empty frame
bad: red artifacts
mozregression --good 2019-04-09 --bad 2019-08-09 --pref gfx.webrender.all:true security.sandbox.content.level:0 -a https://codepen.io/j4k0bkg/pen/bGGpxjY

4:03.18 INFO: Last good revision: f28f80e2466839e5b50c9b9f728988044fecb186 (2019-05-09)
4:03.18 INFO: First bad revision: 2abcefb31ba7b2a1c573edc5695b772826c6a078 (2019-05-10)
4:03.18 INFO: Pushlog:
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=f28f80e2466839e5b50c9b9f728988044fecb186&tochange=2abcefb31ba7b2a1c573edc5695b772826c6a078

I can't get it more precise. 1 year old autoland builds have been deleted 2 weeks ago.
This one mentions "transform":

73254a69497b209b372ed209ce9d004080ea8052 Dzmitry Malyshau — Bug 1532174 - Repace WR RelativeTransform with CoordinateSpaceMapping, improve flattening semantics. r=gw

The bug assignee is inactive on Bugzilla, so the assignee is being reset.