Currently, gtk_widget_set_colormap() has a g_return_if_fail() if the widget is already realized.

But sometimes you want to change the colormap of a realized toplevel. In particular, if you have alpha-window effects (like a transparent terminal),
you don't want to use a argb window unless there is a compositor, since that will use excess video memory. [using an argb window forces it to be redirected, compositor or no]. But you do want to handle a compositor being started on the fly.

Getting this right in your application is *very* hard. You need to transfer:

 - your current size
 - your current position
 - your current desktop (GDK makes this close to impossible because it clears _NET_WM_DESKTOP on map)

From the old window before unrealize(), to the new window after realize(). (The state - maximized, sticky, etc, also needs to be transferred, but that is done already in GtkWindow.)

A set of patches that I'll attach to the gnome-terminal bug 564648 in a moment gives the general flavor of doing so. We should make gtk_widget_set_colormap() just do the right thing.