Affected versions

• 81.0a1 (20200727203201)
• 80.0b1 (20200727193419)
• 79.0 (20200720193547)
• 78.1.0esr (20200722151235)

Affected platforms

• Windows 10x64
• macOs 10.12
• Ubuntu 18.04

Preconditions

• Make sure gfx.webrender.all is set to true.

Steps to reproduce

1. Open Firefox and go to https://bug1358102.bmoattachments.org/attachment.cgi?id=8867531.
2. Change from Layouts to Sliders and observe the behavior.

Expected result

• No flickering occurs.

Actual result

• White flickering while transitioning.

Regression range

• Reproducible with Firefox 74.0a1 (20200206214720). I will search for one ASAP.

Notes

• Attached a screen recording.

2018-08-01 (Fx63) seems completely fine. Then we got two bugs and one was fixed.

good: 2018-08-01 (Fx63) seems completely fine.
bad: 2018-10-01 (Fx64): Submenu was buggy (misplaced text and cut-off elements), but sub-submenu did not have white flickering.
mozregression --good 2018-08-01 --bad 2018-10-01 --pref gfx.webrender.all:true -a https://bug1358102.bmoattachments.org/attachment.cgi?id=8867531

5:25.67 INFO: Last good revision: fea371cafd2c73be689184ca6670e33c3fd1636c (2018-09-10)
5:25.67 INFO: First bad revision: 1169e8a4ca2b7f3cbffdaf70f6d18a5142ed32d7 (2018-09-11)
5:25.67 INFO: Pushlog:
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=fea371cafd2c73be689184ca6670e33c3fd1636c&tochange=1169e8a4ca2b7f3cbffdaf70f6d18a5142ed32d7

This bug:
good: above bad
bad: 2018-12-01 (Fx65): Submenu was buggy (misplaced text and cut-off elements) and sub-submenu did have white flickering (comment 0).
mozregression --good 2018-08-01 --bad 2018-12-01 --pref gfx.webrender.all:true -a https://bug1358102.bmoattachments.org/attachment.cgi?id=8867531

5:56.27 INFO: Last good revision: 12cc80a0e9968ade961879ee07effb815da691f0 (2018-11-08)
5:56.27 INFO: First bad revision: d3d642b624886729636c3690a806f38b4d737731 (2018-11-09)
5:56.27 INFO: Pushlog:
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=12cc80a0e9968ade961879ee07effb815da691f0&tochange=d3d642b624886729636c3690a806f38b4d737731

bad: above bad
good: 2019-05-01 looks like today: Submenu is no longer buggy, but sub-submenu has white flickering (comment 0).
mozregression --find-fix --bad 2019-02-01 --good 2019-05-01 --pref gfx.webrender.all:true -a https://bug1358102.bmoattachments.org/attachment.cgi?id=8867531

4:02.35 INFO: First good revision: 47048ef82b50e3130220d86b09f5addf0b58906e (2019-02-12)
4:02.35 INFO: Last bad revision: b9187fa10f13a7b84f21c973d33d2fdb0f37bbb0 (2019-02-11)
4:02.35 INFO: Pushlog:
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=b9187fa10f13a7b84f21c973d33d2fdb0f37bbb0&tochange=47048ef82b50e3130220d86b09f5addf0b58906e

I can repro this locally, but probably won't have a chance to investigate this week.

Dzmitry, would you have time this week to take a look? Otherwise, I could investigate next week.

I haven't got time to look at it this week, will keep it on the radar for the next one.
From the gifs, it looks to be related to 3D and plane splitting.

Interestingly, the behavior has changed now, and it looks like this.
The only related change I know to this that has come in recently is of bug 1657690

Confirmed, bug 1657690 comment 4 changed the behavior from comment 0 into comment 4.

Spatial tree of a WR capture. The relative transform from node 7 to node 2 shows up to be front-face visible, were we are expecting it to be back face.

When we are looking at a transform of an element relative to the surface
it's rendered into, we used to only consider the final transformation
when determining back-face visibility.
However, in case there are any Flat reference frames on the way,
the transformation is flattened. We should be checking for back-face
visibility on each such step.

This doc was helpful, although it doesn't have all the answers:
https://docs.google.com/document/d/1yb4a_uhTG3KmcbGPta4B9p67v1HO_qY8ZMk-otGOwTc/

https://hg.mozilla.org/mozilla-central/rev/3013857e5351

Verified with Firefox 83.0a1 (20201012152719) with WebRender on Windows 10x64, macOS 10.12 and Ubuntu 18.04.