> Modal windows prevent interaction with other windows in the same application. To keep modal dialogs on top of main application windows, use gtk_window_set_transient_for() to make the dialog transient for the parent; most window managers will then disallow lowering the dialog below the parent.
The comment links to the documentation, which says it explicitly...
And then, many other places in the documentation, e.g. https://docs.gtk.org/gtk4/method.Window.set_modal.html
> Modal windows prevent interaction with other windows in the same application. To keep modal dialogs on top of main application windows, use gtk_window_set_transient_for() to make the dialog transient for the parent; most window managers will then disallow lowering the dialog below the parent.