Slightly more synchronous completer?

Is it possible to make the completer slightly more synchronous. I appreciate that it doesn't block, but sometimes I like to type tab and then more text, or enter, for cases where I know the completion is unique, so that it will fill in the common part automatically. But if you type any text before the completion code finishes, the completion is cancelled and doesn't happen.

I'm probably asking two contradictory things, having it be both nonblocking and synchronous. But maybe instead of an immediate cancel it could use a timeout, so that it could block for a fraction of a second (the actual ideal time would need to be found experimentally), but then be nonblocking after that to avoid issues where the completer hangs.

Start by locating the completer's cancellation and input-handling entry points, then reproduce the case where typing more text or pressing Enter cancels an unfinished completion. Compare possible timeout behavior with the current nonblocking behavior; done requires a documented, tested choice that preserves responsiveness while allowing the intended completion to finish.