I was fiddling around with key-mappings, and when I mapped an action to Alt+S, it entered Screenshot mode. And of course, now the Screenshot mode binding has been removed and I can't get out of it. It's okay, I fixed up the keybindings.json file, but in general, I think already-bound keys should not do their assigned actions in this dialog.
Additionally, if I click on more than one key binding without entering new key combinations, all the selected fields will display '???'. Fortunately, only the most recently clicked binding will take what I press and the others reset.
While in-game: 1) Hit Esc to bring up the menu 2) Click on Controls 3) Choose any action except ToggleScreenshotMode to map 4) Enter the same key combination as ToggleScreenshotMode