493dd5d93c caused the `::before`
pseudo-element node to be inserted before the element's content, which
caused issues with how we determine where to insert inline nodes into
the layout tree. At the time, I noticed the issue with contents of flex
containers, and prevented them from merging into a `::before` box.
However, a similar situation happens when we're not in a flex container,
but the pseudo-element has `display: block`. This commit fixes that
situation by using the same logic in both places, so a similar mistake
can't be made again.
This fixes the tab text being invisible on GitHub project pages. :^)
