Just following up: reading Kevin's suggestions reminded me that I'd added this ArgDown extension yesterday ( https://marketplace.visualstudio.com/items?itemName=christianvoigt.argdown-vscode ). I don't think it started causing any trouble until I reloaded the window, so I'd already forgotten about adding it when I messaged. VS Code is behaving so far today, now that I've disabled the ArgDown extension. Maybe it's another one to add to the list of incompatible / troublemaker extensions?