Hi,
In bug 1374247, I intend to remove the XBL compatibility hack introduced
in bug 653881 [1] for which <xbl:children> elements may be "transparent"
in selector-matching.
That means that a selector like .foo > .bar, would match a tree like:
<div class="foo">
<xbl:children>
<div class="bar"></div>
</xbl:children>
</div>
The motivation is the following: This was not implemented in stylo, but
the Firefox UI depends on it, so we need to do something if we want
stylo on chrome documents.
I fear that it is the kind of thing that if we ever implement, it will
stay there forever. Also, one-offs like these tend to be buggy and
overlooked when implementing optimizations.
Finally, this hack doesn't apply to Shadow DOM, so removing it would
likely help with the efforts of transitioning away from XBL.
Since just removing it would have chances to silently break parts of the
Firefox UI, I'm landing a diagnostic assertion to verify that we don't
use it. I've been browsing for a couple days now with it without issues,
but if you get a crash pointing you to that bug, please file and I'll
take care of it :)
Thanks!
-- Emilio
[1]:
https://hg.mozilla.org/mozilla-central/rev/1b8207252e9ca1194eccd3adc14b961785fb4e8e
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