Thank you very much, Michael!

Yes, I keep the browser inside.

It works again! ...kind of...
So it does hide the UI, effectively enlarging the current window scheme to full screen, however:

On the first activation it will not hide the UI, but only closes up any expanded open tool options.
On the second activation it will hid the entire UI.

Then after that will will then both hide the UI and close any expanded tool option simultaneously.