Sequence of events in previous scenario:

1. car.draw throws an error
2. The wrapping call_protected invokes send_errors_to_output
3. send_errors_to_output uses Text.insert_text, which was extracted from the paste implementation on Nov 20. But it doesn't scroll, and so the cursor is off screen.
4. Drawing frames never sets cursor_x or cursor_y.
5. Mouse wheel indirectly requires them to be set. Boom.