327f1b06be
This is almost a revert of bug 1741089 (just with the relevant test fix
and so on). With the previous patch, for something like:
<menupopup>
<menuitem />
</menupopup>
We would still generate the frames for the menuitem. That causes various
a11y tests to fail and starts loading images used by the menuitem which
was caught by browser_startup_images.js.
Make the children completely invisible to layout by keeping them
unslotted. This makes getComputedStyle on them return an empty style,
but only one test was depending on that.
Depends on D134331
Differential Revision: https://phabricator.services.mozilla.com/D134606