gdk:toplevel-size-bounds (size)

size -- a gdk:toplevel-size instance
width -- an integer with the width
height -- an integer with the height

Retrieves the bounds the toplevel is placed within. The bounds represent the largest size a toplevel may have while still being able to fit within some type of boundary. Depending on the backend, this may be equivalent to the dimensions of the work area or the monitor on which the window is being presented on, or something else that limits the way a toplevel can be presented.