Artist-mode on an empty buffer.
C-c C-a C-o rectangle <RET>,
<RET> to start drawing a rectangle. You can go right and left, but you can't go down. On a non empty buffer, you can actually go up or down, but not past the last line of the buffer, at least with the keyboard. The question is how do you draw a rectangle with the keyboard even on an empty buffer ?
video illustration : https://vimeo.com/491189784