The "CRT filter: on / off" of a game's options. The game is shown through two effects: a crt() for the whole game, in createGame's post chain, and a dither() the scene puts on with usePostProcess, down to the Mega Drive's eight steps a colour.
F turns every effect off at once, the game's and the scene's, with useGame().setPostProcessEnabled. D turns only the dither off, with its own enabled. Nothing is forgotten: turn the dither off, then everything off and on again, and the dither stays off.