Comparing my upgraded installation of xwiki 9.11.4 to a brand new, empty installation of xwiki 9.11.4 - the display doesn’t look quite right, especially when using panels.
Any idea what could be causing this? Changing the panels size doesn’t help, but the panel will disappear correctly if I drag the page smaller. I’ve also tried reinstalling the panels app which didn’t help either.
Indeed 
Did you try to force clean your browser cache?
Yes, I did! I’ve also tried three different browsers.
This issue ended up being caused by a custom StyleSheetExtension.