I haven't spotted any UI regression from this, and this should generally
make the XUL -> modern flex transition easier, and simplify some
of the relevant code.
This does fix a few layout issues with emulated flexbox.
For the most part, this shouldn't change behavior without that. This
changes behavior if you have mixed inline/non-inline content in the same
XUL box (before they'd get a single item, now you'd get the flexbox /
grid behavior of one item per inline run), and multiple inline-elements
(which would become their own flex items). But I pushed a patch with
some asserts and they didn't fire on our browser mochitests, so I think
we're good.
The UA rule refactoring (removing the inherit from xul anon blocks)
shouldn't matter in practice, since we only have one item (so
box-ordinal is irrelevant) and they have overflow: visible (so
text-overflow and overflow-clip-box shouldn't have an effect).
Differential Revision: https://phabricator.services.mozilla.com/D156375
b5c2fd645c