If a custom element with a button and a default slot inside its shadowdom is mounted with visibility: hidden, then even after the visibility update, the assigned textnode for the button's label is ignored. Demo: https://codepen.io/istutto/pen/xxMPrZK
<rdar://problem/118496530>
I believe this is fixed by: https://bugs.webkit.org/show_bug.cgi?id=264410
Thanks for confirming this is fixed!