I have found that it can turn suddenly itself on.
1. I think I see what happened.
Every time you enter the message text box, you get that &%#@! prompt.
My first Enter disposed of the box, turning on Visual mode.
2. Something weird is going on with right clicking. I use Firefox, which has a spelling checker going for text entry areas. Right clicking on a misspelled word (to see a list of possible good spellings) did not act look/right.
ESCaping out of the menu, I tired again and got what I expected.
3. Ditto Selection when entering the box.
After previewing my message, I went to the message text area and highlighted a select. Upon releasing the mouse button, I got the "Visual Mode" prompt box, and clicked cancel.
My selection (highlighted text) had changed, expanding at both top and bottom so that the original selection was now in the middle of the highlight.