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. :^)
