Expose events and unrendered chunks of screen

["Lewin A.R.W. Edwards" <larwe larwe com>]

I'm having yet another "fit and finish" type problem, on which I'd 
really like some advice because there seems to be an underlying problem 
I don't understand.

In my program, I have a 1024x768 window covering the screen. The Xserver 
runs with a virtual res of 1024x769 due to a hardware weirdness. I 
handle expose_events for the window by blitting a window-sized pixbuf 
onto the window area directly.

Now take a look at this snippet from my startup code:

        // Initialize windowing library and hide cursor
        gtk_init(&argc, &argv);
        gdk_rgb_init();
        screenX = gdk_screen_width();
        screenY = gdk_screen_height();

        useless.red = useless.green = useless.blue = 0;
        useless.pixel = 0;
        mainwindow = gtk_window_new(GTK_WINDOW_POPUP);
        gtk_widget_set_usize(mainwindow, screenX, screenY);
        gtk_widget_set_uposition(mainwindow,0,0);

        gtk_widget_show_all(mainwindow);
        gdk_window_set_background(mainwindow->window, &useless);

        // ... some internal code to set my entire offscreen buf black

        // copy buffer to window
        XPEG_RenderDisplay();

        // COMMENT1
        while (g_main_iteration(FALSE));
        // COMMENT2
        XPEG_RenderDisplay();
        // COMMENT3
        while (g_main_iteration(FALSE));

XPEG_RenderDisplay() just blits my offscreen buffer onto the window. 
It's the same function I call for expose_event.

If I omit the lines from COMMENT2 onwards, when my program starts up it 
will flash a (correct) black window, then the window will fill with 
light gray. I need to have those extra two lines to fill the window with 
black again and wait for the event queue (?) to empty.

Secondly, I sometimes call an external program that switches the display 
down to 640x480 and runs some video. When this program exits and my 
program returns, a 640x480 hole in the middle of my window is light gray 
despite the fact that I handled the expose_event. (And I put a trace 
message in there and yes the expose_event is being handled by my code - 
but something ELSE is called after me).

It seems as if something "on top of" my window has an undesirable 
default behavior. How can I override this?

--
-- Lewin A.R.W. Edwards (Embedded Engineer)
Work     http://www.digi-frame.com/
Personal http://www.larwe.com/
Tel (914) 937-4090 9am-6pm ET M-F