Inset box shadow rendering bug

[nc4rrillo]

While browsing this docs.microsoft.com page I noticed strange lines in the tree view on the left.

If you look at this page you'll notice extra lines being drawn for inset box shadows compared to FF Nightly with WR off.

Setting any of the values to non-zero causes the issue to go away.

[nc4rrillo]

Tested this on Linux with an Intel GPU and on Windows 10 with an nVidia GPU. Seems to be affected by the page zoom in FF.

[nc4rrillo]

I've tried to recreate this in a sample app but I can't trigger the weird behavior, wondering if its Gecko specific or if I have the conditions that trigger it wrong.

[nc4rrillo]

bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1419528

[Darkspirit]

This is a regression from Nightly's perspective: https://bugzilla.mozilla.org/show_bug.cgi?id=1419528#c2
(a624aa6...7892f53)

(Radeon RX480 @ Debian Testing, KDE)

[glennw]

These glitches often depend on the contents of the surface cache and the allocation patterns, so it might be quite difficult to hand-craft a test case. What I normally do in these cases is do a binary wrench recording (export ENABLE_WR_RECORDING=1) from Gecko - and then replay this file in wrench. If we can reproduce it that way, it's normally quite easy to reduce the test case and fix from there.

[glennw]

I think this is fixed by #2307 - are you able to confirm?

[nc4rrillo]

Yes can confirm that fixed it.