View the attached testcase on a non-hidpi screen.

The main test element here is just over 16M pixels tall. Scroll to the very bottom, and observe the erratic rendering of the last line -- glyphs are shifted up/down from the proper baseline, and the glyph bitmaps may be broken (e.g. stretched or squashed by a pixel).

The effect also occurs on a Retina screen, though it is a bit less obvious; but there it begins half-way down the document, implying that the critical height is 16M device pixels, not CSS px. (Similarly, on a Windows display with 150% scaling, the corruption begins about 2/3 of the way down the document.)

Mozregression points to https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=02b89c29412b6c1444fe32a4847e5261e2bb3d00&tochange=2ccc6648064315964dd23039ad28ebf7d9f82999 as when this started happening.

Of the patches in that range, I wonder if Bug 1545006 - Use external scroll offsets to move primitives and clips into local space might be the issue?

Really interesting test case. I can reproduce on windows.

Glenn do you have an idea why this might happen?

Seems like a dupe of bug 1667534 and bug 1629859.

I got a different, later regression range: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=a35461d1fc07e6e855d064453363a46d32bb5a3d&tochange=bd7c35b7e234712f94f6dbb755ad983c1db8b07b. I'm not able to spot an obvious candidate regressor.

Can't repro when setting layers.async-pan-zoom.enabled:false!

Does this occur on non-Windows platforms and/or with gfx.webrender.compositor disabled (restart required)?

(In reply to Glenn Watson [:gw] from comment #8)

Does this occur on non-Windows platforms and/or with gfx.webrender.compositor disabled (restart required)?

Yes.
Gnome Wayland, Debian Testing, Intel Iris Graphics 6100 (BDW GT3), 2560x1440
STR: Zoom to 500% and press End to scroll to the bottom.

Reproducible with:
mozregression --launch 2021-08-28 --pref gfx.webrender.software:true -a https://bugzilla.mozilla.org/attachment.cgi?id=9236904
mozregression --launch 2021-08-28 --pref gfx.webrender.all:true -a https://bugzilla.mozilla.org/attachment.cgi?id=9236904
MOZ_X11_EGL=1 mozregression --launch 2021-08-28 --pref gfx.webrender.all:true -a https://bugzilla.mozilla.org/attachment.cgi?id=9236904
MOZ_ENABLE_WAYLAND=1 mozregression --launch 2021-08-28 --pref gfx.webrender.all:true -a https://bugzilla.mozilla.org/attachment.cgi?id=9236904
MOZ_ENABLE_WAYLAND=1 mozregression --launch 2021-08-28 --pref gfx.webrender.all:true gfx.webrender.compositor:true -a https://bugzilla.mozilla.org/attachment.cgi?id=9236904

Not reproducible:
with layers.async-pan-zoom.enabled:false

Can confirm that layers.async-pan-zoom.enabled = false also avoids Bug 1673939 with Fission disabled. Since this bug is very similar and has the same regression window per Comment 2, I think it's fair to say that it is also regressed by Bug 1545006.

Unassigning for now, as I'm not actively working on this right now.

The severity field for this bug is set to S4. However, the following bug duplicate has higher severity:

• Bug 1667534: S3

:bhood, could you consider increasing the severity of this bug to S3?

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