You send the key event and THEN start the debounce timer. I think most people tasked with writing a debounce function wait for the key state to change, then start a timer for how long they think the switch will bounce for, check the state after the timer expires, and then send the key event. Obviously, that adds latency. But what you can do instead is send the key event immediately and then completely ignore events during the debounce interval.

Simple, you just ignore further input for x milliseconds after the first state change, where x is slightly less than the fastest supported double-click interval.

You certainly don't want to low-pass filter the whole thing, or otherwise wait for the contact to settle before reporting the keystroke or mouse click.