Really silly problem here.
In Visual Studio 2010, the text cursor has changed from the blinking line, to a blinking grey box around the characters. When I type over
If you don't have an insert key, and you're using Visual Studio 2019, then double-clicking the OVR text in the bottom right corner does not work. You'll have to use an on-screen keyboard, if you have one of those, or figure out what your insert key is mapped to. For me, on my mac keyboard hooked up to windows 10, it is the 0 key on the keypad.