To wait while callbacks are being handled, use one of:

     handle_events(-ExitCond, +Context)

handle_events will enter a failure-driven loop listening for events on all open ProXL displays, and executing callbacks. It exits when either any of the callbacks binds its exit variable to a value that unifies with ExitCond, or when all ProXL Windows that have associated callbacks have been destroyed (in which case ExitCond will be unified with none).

If the Displayable is omitted, the default Display is used.

If ExitCond is omitted, it defaults to none.

Context is passed to any callbacks that want it, and may be used to cause callbacks to behave differently in different contexts. If omitted, it defaults to an anonymous variable.

For backward compatibility,

     handle_events(+Displayable, -ExitCond, +Context)

is equivalent to handle_events(ExitCond, Context). The Displayable is ignored.