Okay, that was a little unexpected. The old code doesn't show the effect, but the new code produces the wrong-buffer-rendering effect only if the OpenGL window is sitting over the Konsole window (and only when the Konsole window is changing (such as printing out debugging text to try and figure out why it's flickering)).

So, it seems like the problem is probably with the handling of resize/expose events. I changed the PyGame handling of those, but I haven't yet figured out how that change could have generated the behaviour.

Anywho, at least I now realise that it's not the swapping code at fault... back to my debugging now...