The left firefox window is gfx.e10s.font-list.shared is false, the right one is gfx.e10s.font-list.shared is true, you can see font differences all over the places.

The regression range by mozregression is https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=184436f571870db956c30f75c44c82f24ec36eba&tochange=1a8269aa3f6e27978920d9ba7f40367548e7aee0, it includes bug 1619349 and bug 1619350, I'd bet it's bug 1619350.

Yeah, that's definitely not expected! Clearly need to fix this before we can consider flipping the pref.

Ugh - looks like the merge to beta is currently happening, but this didn't make it from autoland to central in time to catch the train. So we'll need to uplift in order to avoid breakage for people trying to test the shared-fontlist option.

Comment on attachment 9138314 [details]
Bug 1627397 - Fix broken behavior of gfxPlatformFontList::FindAndAddFamilies when shared fontlist is enabled. r=jwatt

Beta/Release Uplift Approval Request

• User impact if declined: Broken font rendering (browser uses a "random" wrong font) for users who enable the shared-fontlist pref
• Is this code covered by automated tests?: No
• Has the fix been verified in Nightly?: No
• Needs manual test from QE?: No
• If yes, steps to reproduce: Verified bug and fix in local builds.
  STR: set gfx.e10s.font-list.shared to true, restart the browser, and check whether fonts look normal.
• List of other uplifts needed: None
• Risk to taking this patch: Low
• Why is the change risky/not risky? (and alternatives if risky): Trivial fix; only touches code that is specific to the shared-fontlist pref, so no impact on users with default config.
• String changes made/needed:

https://hg.mozilla.org/mozilla-central/rev/dde14cda50ba

Comment on attachment 9138314 [details]
Bug 1627397 - Fix broken behavior of gfxPlatformFontList::FindAndAddFamilies when shared fontlist is enabled. r=jwatt

Not strictly needed because the affected code is preffed off by default, but won't hurt to take for those testing it out. Approved for 76.0b2.

https://hg.mozilla.org/releases/mozilla-beta/rev/2de6dd1b0452

I've reproduced the issue with Fx 76.0a1 (2020-04-03) on Windows 10.
The issue is verified fixed with Fx 77.0a1 (2020-04-07) and Fx 76.0b2 on Windows 10 and Ubuntu 18.04.