Related events

'expose_event' is generated when the window or a portion of it has become visible and needs to be redrawn. Simply draw the offscreen pixmap you've so thoughtfully created to the window and be done.

'configure_event' is generated anytime the window is resized. You don't necessarily have to handle it, but then the size of your offscreen pixmap and onscreen window may not match up.

Word of caution: be careful about creating too many unnecessary pixmaps, as the old ones do not get destroyed in the current version of PHP-GTK. Either disallow resizing the drawing area or create a big pixmap upfront and use only a portion of it.