Some text has black boxes around the glyphs

[mstange]

https://bugzilla.mozilla.org/show_bug.cgi?id=1407345

That bug has a testcase which uses mix-blend-mode, but @gankro tells me he has also seen this issue with 3d transforms.

[Gankra]

Mine was https://bugzilla.mozilla.org/show_bug.cgi?id=1406179

data:text/html,<div style="perspective:1000px"><div style="transform-style: preserve-3d">hallo

[mstange]

After #1816, the testcase with mix-blend-mode now gets black boxes in both lines, not just in the second line.

[glennw]

OK, I can see the problem - we don't determine early enough that this run is on a transparent background and disable subpixel AA. Even though in the near future, we'll support that, I'm going to do the work to be able to determine earlier when a run is on a transparent background - because this is part of the planned work in #1774 anyway, that will be useful to have for several other reasons.

[glennw]

I added an interim fix in #1844.

[mstange]

This testcase still shows the issue in Firefox with an updated webrender. I'm not sure if Firefox actually forwards the CSS isolation property as an isolated WebRender stacking context.

[kvark]

Looks like we need a reftest for this