Every once in a while when editing a formula, I will go to add a parenthesis at the beginning of the line and try to type (shift-9) to get a "(". However, I will accidentally press shift-8 instead and the result will be to convert the line to some weird rich text and place a small solid red square at the beginning of the line.

I don't know what that is for, but I never want to do it and I want to disable whatever is going on; it effectively means I have to delete the line and start again because I don't know how to undo it. In fact it's happened for a long time and I've only just figured out what was actually happening. I tried looking at keyboard shortcuts online but didn't find anything for "shift-8". This only happens at the beginning of the line.

Any suggestions how to disable this feature would be appreciated.