I've just been typing HTML in a browser tab online and all of a sudden had triggered insert mode (where typing overwrites the following character) using a keyboard on a Chromebook (Chrome OS) - I had no idea how to switch it off so just had to save my draft. I never knew I had this capability and can't figure out how to get it back on. The shortcut lists online offer no hints!
I think I was playing with ctrl shift and alt but no idea what other keys unfortunately...
The search key on the chromebook (the one represented by the magnifying glass) when pressed in combination with the period key will engage and disengage overwrite.