The previous code tried to do it, but it did it wrongly, as the overflow clip
comes from the parent, not the child.
Thus when we change a style that influences it, we weren't invalidating the
SIMPLE_DISPLAY_LIST bit, and such.
Make the reftest that caught this fail more reliable.
Differential Revision: https://phabricator.services.mozilla.com/D51805
--HG--
extra : moz-landing-system : lando
0d3b6b9453