The hand waved from a window is already speaking across a growing distance.

Goodbyes begin before the leaving does. By the time a hand goes up at a window, the ordinary way of speaking has already gone, and a gesture is doing work that a sentence was doing five minutes earlier. That is why waving feels both small and enormous: it is the last available form of contact, stretched as far as it will reach. Distance changes the language before it changes anything else.

If someone is leaving soon, say the important thing while the room still allows sentences. Waving is a fine ending and a poor place to start.