A quick one: the button for closing the game was hard to recognise. It now simply says QUIT GAME, at the very bottom of the main menu, and closes the game after a short confirmation. Your progress is saved as always.
Thanks to everyone who pointed it out.
Posted by the game’s developer or publisher to its Steam community hub and published by Valve through Steam’s public news API. Sifter reproduces it unedited and writes none of it.