X11 windows without window manager

[Robert Latest <boblatest gmail com>]

Hello all,

I'm working on a simple app that will eventually run full-screen on an
industrial touchscreen controller (Linux/X11) without a window
manager. My question is how to do things that are normally handled by
a window manager, especially opening additional windows (dialogs) on
top of the main window.

I have tried this on my Linux box, and it works like expected, but
since I'm not yet familiar with the target system I don't know if this
is just an accident and everything will break when I eventually deploy
my app.

Short question: What is, for example, the canonically correct way of
creating a centered window on top of the main window? Can I rely on it
being visible (and not behind the main window)? Can I rely on it being
centered and not shoved in some corner?

I've looked at the GdkWindow API, but all functions pertaining to such
operations merely "ask the window manager" to resize/move the window.
Do I have to go more low-level than libgdk?

The alternative way I see is to keep everything inside the top-level
window and use a notebook with invisible tabs to maintan a stack of
widgets. It works, but it is harder to manage than showing or hiding
windows, or calls to gtk_dialog_run().

Thanks, and best regards,
robert