Random viewer selection modal invisible on fullscreen presentation

[hiroshisuga]

Describe the bug
The random viewer selection modal is invisible when the presentation is in the fullscreen mode.

To Reproduce
Steps to reproduce the behavior:

1. in the viewer's screen, set the presentation full-screen by clicking the bottom-right button
2. start the random viewer selection
3. the modal not visible

Expected behavior
the modal visible

Actual behavior
invisible

BBB version (optional):
the latest 2.3a

Additional context
related to #10783, which has been fixed by #11319 .
I tried to do the same as #11319 but the modal container seems to get back immediately to the original place, staying invisible.