STR:

1. Start with gfx.webrender.all = true on Ubuntu 19.10.
2. Use a magnifying glass to inspect fonts in the UI, eg tab title, location bar, sidebar etc.

UI text is using grayscale and not subpixel anti-aliasing. Web content is unaffected.

Regression window:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=04e8766a29242d4deae31b5b04e6ac61ebf61ffd&tochange=9b42bdc4889fe7782df9b2a0aa990ed5e62cb04c

Regressed by Bug 1584439.

Glenn, your patch bug 1584439 caused this regression, could you have a look please? Thanks.

This is related to https://bugzilla.mozilla.org/show_bug.cgi?id=1575743, which I'll mark as a dependency for now. It's not totally clear whether we can fix it by making the changes suggested in that bug, or if we'll need to make some further internal changes to WR to try and support this case. I'll follow up on the bug above, and we'll work out the right way to solve this.

nothing to do here for 71, since WR isn't shipping on linux yet

There's ongoing discussion in 1575743 - but it seems like subpixel AA is working again on nightly on Linux + WR (see https://bugzilla.mozilla.org/show_bug.cgi?id=1575743#c24 for more info). Is it working for you on nightly? (Even if it is, I still want to find out exactly why it changed).

This has been fixed, albeit accidentally. Bug 1575743 is being left open while we work out a way to add a regression test for this in future. Please re-open if you're still seeing the issue (I confirmed today that it's working on Linux + Windows with current m-c).

I'm seeing this again in Firefox 76 on Linux. Furthermore, even with WebRender disabled, for some weird reason the Tree Style Tab sidebar is also using grayscale antialiasing, even though the History sidebar correctly subpixels. :-/

I have this issue on Linux, Firefox 90.0.2. No subpixel anti-aliasing in tab bar, address bar, sidebar, bookmark bar when WebRender is enabled (this is default).
I've made a bug with screenshots.
https://bugzilla.mozilla.org/show_bug.cgi?id=1724057