Bug 1704954 introduced a mitigation for bug 1638709, disabling DirectComposition for some users. However, the mitigation conditions are overbroad, and the fallback path has exposed another highly visible issue in bug 1763981.

We should allow users to disable the mitigation in question, to provide at least those not affected by bug 1638709 with a persistent workaround for bug 1763981.

Bug 1638709 describes artifacts when the webrender compositor is enabled
on certain Windows systems with NVIDIA cards and mixed monitor refresh
rates. As of bug 1704954, we currently disable DirectComposition on such
systems in Beta and Release as a mitigation.

Unfortunately, on a significant (albeit not yet fully characterized)
fraction of Windows 11 machines, the fallback compositor also has
issues, resulting in bug 1763981 (clipping of web content on first
entering fullscreen). Users experiencing this latter issue have hitherto
not reported the former when testing on Nightly, suggesting that the
mitigation may be getting applied overbroadly.

Add a pref to allow disabling the mitigation in question, so that users
experiencing bug 1763981, but who are not subject to 1638709, have a
less intrusive workaround than remembering to hit F11 twice every time
they start Firefox.

Backed out for causing build bustages on gfxConfigManager.cpp

• Backout link
• Push with failures
• Failure Log
• Failure line: /builds/worker/checkouts/gecko/gfx/config/gfxConfigManager.cpp:265:20: error: no member named 'gfx_webrender_dcomp_apply_1704954_AtStartup' in namespace 'mozilla::StaticPrefs'; did you mean 'gfx_webrender_compositor_AtStartup'?

https://hg.mozilla.org/mozilla-central/rev/efa37af06091

Comment on attachment 9316905 [details]
Bug 1816001 - allow users to disable mitigation for bug 1638709 r?#gfx-reviewers

Beta/Release Uplift Approval Request

• User impact if declined: Users on Release will continue to have only an annoying workaround for bug 1763981 for the next month.
• Is this code covered by automated tests?: No
• Has the fix been verified in Nightly?: Yes
• Needs manual test from QE?: No
• If yes, steps to reproduce:
• List of other uplifts needed: None
• Risk to taking this patch: Low
• Why is the change risky/not risky? (and alternatives if risky): This change adds a single pref and, if that pref is changed, disables a single block of code which is already always disabled on Nightly.
• String changes made/needed:
• Is Android affected?: No

Comment on attachment 9316905 [details]
Bug 1816001 - allow users to disable mitigation for bug 1638709 r?#gfx-reviewers

I don't think this is a good candidate for the dot release as this is a long-standing issue since 89, we ship 111 with this patch in 2 weeks and we prefer not to ask end users to touch prefs in about:config on the release channel. This would also need a support article for the dot release shipping day. Let's have it ride the 111 train, thanks.