When Zotero is not in full screen, I sometimes observe this:
https://s3.amazonaws.com/zotero.org/images/forums/u265723/30nevp7hah7rx0d2p7cg.png

The vertical size of the List all tabs window seems to depend on the size of the Zotero window on the screen and its position.
For a specific size and position, I saw the problem only for 1 to 3 tabs opened.

It may be related to this:
https://forums.zotero.org/discussion/124296/tabs-menu-blinking-when-opening

Zotero 7.1-beta.46+5e1541c5e (64-bit)
Windows 10