Updating background pixmap?

[Martin Craig <mcra98 esc cam ac uk>]

Well I posted this question before but no-one replied (sniff sniff...)
so here it is again, perhaps in a slightly clearer form. I have
a text box, to which I have applied a background pixmap
(style->bg_pixmap = my_pixmap, gtk_widget_set_style, etc).
However when I draw onto it (using gdk_draw_line or whatever)
it doesn't change until I cover & uncover it a few times. I've
also tried sending the expose event explicitly.

I need a clean way to make sure it gets updated. It doesn't need 
to be ultra-fast, just reliable. Is this possible? Is there a better
way to draw onto the background of a text box? 

If this is in the documentation somewhere obvious, sorry, but 
please tell me where!

Thanks, Martin.