On Meta-Wiki i get a weird effect if i make my browser window small enough. On https://meta.wikimedia.org/wiki/Main_Page/de the part of the tabs "Lesen Übersetzen Versionsgeschichte" starts bouncing, if the window is so small, that the tabs normally would collapse one of them.
Windows 10, Firefox 81.0.2 (64-Bit) This happens also with older firefox versions and with MS Edge
Steps to Reproduce:
Goto Meta-Wiki. Use vector-skin. Make your browser window small enough until bouncing starts.