This probably deserves a comment as of why it belongs to this bug.
This patch series caused a single, reproducible timeout on
browser_ext_themes_toolbars.js, where the transitionend event it awaits for
stops triggering.
I got fascinated by it and I decided to poke around it in rr instead of just
removing the await line, and here's what's going on.
In the previous implementation of _sanitizeCSSColor, we were not flushing style
because of the optimization bug 1363805 introduced (which wasn't supposed to
deal with out-of-document elements, but it accidentally did so).
In any case, the fact that we were not flushing style in _sanitizeCSSColor
caused us to flush style sometime later when the lwtheme attribute was already
set up, and thus the selector in here matched:
https://searchfox.org/mozilla-central/rev/cfaa5a1d48d6bc6552199e73004ecb05d0a9c921/browser/themes/shared/browser.inc.css#40
And thus caused the transition rule to apply at a time where the
background-color change happened.
Now we were flushing on getComputedStyle on every call, and in the most
inefficient way possible (changing a custom property on the root before each
property change, which causes us to restyle the whole document to propagate it
down to all descendants).
Furthermore, we were flushing style at a time where the lwtheme attribute
change had not yet happened, and thus when the background-color changed, there
was no transition rule applicable, and the transition didn't fire.
This patch changes LightweightThemeConsumer to avoid restyling the whole
document over and over.
Also, while at it I realized that you could fool the sanitizer with !important
in an experiment stylesheet or with other !important rule in the page really.
It's not clear why you'd do that, but it may be worth to just making that
function completely sound, so I did that and added a test for it.
Differential Revision: https://phabricator.services.mozilla.com/D13716
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