That's a common problem, a key just can't register a mouse button release outside of itself.

Could it check if it is still in a pressed state? There would be a lag, but it wouldn't be broken. I guess.

I imagine there's several ways to do it, it's a matter of figuring out which one is the most efficient for the situation at hand.

For instance, you could log what key was pressed with its own mouse event, then detect a generic mouse release event not tied to the key, look up the log that recorded what key was pressed, and then react accordingly to that. I wouldn't think it would be the best method for a multi-touch device though.