In particular, this property now interacts correctly when the flex
container has flex-wrap: wrap-reverse.
This caused some "regressions" in WPT tests for negative overflow in
flex containers, but the previous behavior wasn't correct either,
it just happened to give false positives on tests.
...when running in test mode. This cuts down on the time it takes to run
the imported WPT tests, and you can still get the full error by opening
tests in the browser.
1. We were not propagating selectedness updates from option to select
if the option was inside an optgroup.
2. When two or more options were selected, we were always favoring the
last one in tree order, instead of the last one that got checked.
3. We were neglecting to return in the `display size is 1` case when
all elements were disabled.
This was covered by some of the :has() selector tests. :^)
We basically need to do this for every invocation of invalidate_style()
right now, so let's just do it inside invalidate_style() itself.
Fixes one missing invalidation issue caught by a WPT test. :^)
