How to force a window to resize?

["Felix E. Klee" <felix klee inka de>]

Hi,

I am creating a little program that does a computation and updates the
display from time to time. Because I don't want to use multithreading or 
timer objects I have to call
    gtk_widget_draw(mainWindow, &rect);
manually after making changes to the main window. However, when I resize
widgets the size of the window changes only after the computation has
finished and control is given back to the GTK event loop.

How can I force the window to resize itself immediately after changing
its size?

TIA,

Felix