I updated last post: setting height to 100vh doesn’t fully solve the problem, because the fixed element still scrolls with the page (instead of staying in place like fixed elements normally do).
3D transform with
backface-visibility: hidden; sets the viewport of the fixed element to be the transformed element. So it would be better to get rid of that style.
Easy Option #1: move overlay element outside any of the skel-panel elements.
Easy Option #2: edit skel-panel code and remove backface-visibility rule.
Hard Option #3: throw feedback at skel-panel author(s) and tell about this issue and keep doing so until they fix it in their release.