Can I make a fullscreen overlay without the top bar disappearing?

[Dylan McCall <dylanmccall gmail com>]

I'm making an app that displays a transparent, non-interactive
fullscreen overlay to convince someone to take a break without
interfering with what they are doing. So, I want this overlay to
disturb nothing in terms of what the user can currently do: it just
serves as a constant reminder (and makes it harder to see what else is
on the screen).

I have this working both with a popup window and with a toplevel. If I
make that overlay match the size of the screen - either through
Gtk.Window.fullscreen() or, more importantly,
set_size_request(monitor.width, monitor.height)) - Gnome Shell's top
bar disappears. I'm not particularly concerned whether the top bar
appears above or below my overlay, but I would really like it to be
visible. Is there a way I can make that happen?

One might observe I can use Gtk.Window.maximize(), but I can't do that
with a popup, and the toplevel approach demands I do all sorts of
weird things like set_accept_focus(false) to prevent the user from
pressing Ctrl+Alt+Down and unsticking the overlay. And, unless I'm
mistaken, there's no clean way of knowing what dimensions a maximized
window will have. The only thing I can really get my popup to be is
fullscreen or some safe number like 90% of screen height.

This app isn't aiming to be super portable, so I'm willing to do
specific Gnome Shell / Mutter code as long as it isn't crazy :)

Thanks!

Dylan