Steps to replicate the issue (include links if applicable):

• Collapse the read-mode table of contents sidebar (so it becomes the button by the heading / the floating button)
• Edit the page in VisualEditor

What happens?:

The TOC button appears, but opens an empty popup.

What should have happened instead?:

Probably the button should also be hidden, until T380598 is worked on to give in-VE editing a live-updating TOC.

This is presumably a symptom of T379444 not having been worked on yet to provide a proper menu-disabling API, and so the current menu-hiding hack used by VE (T379443) is successfully hiding the menu but doesn't know about the (new?) button.