"Maximizing Tor Browser" Notification can exist multiple times

The following Message that overlays the top part of the browser when maximizing it, can open itself multiple times. See i.imgur.com/hQfxjO1.png

Steps to reproduce:

1. Open Tor (6.0.3)
2. Maximize (message shows up)
3. Revert to other size then maximum / minimum
4. Repeat 2,3

Multiple messages now lie on top of each other which have to be closed individually.

Expected behavior: Only one of those messages can be open at one time.

Trac:
Username: oldmanjohnson