Any keypress which doesn't correspond to a valid label should abort the jump

This is debatable. In some situations one could, to the contrary, enjoy the opportunity to, say, correct their pick with backspace, arrow keys, etc. Or simply get better feedback on why a particular jump didn't happen, e.g. they'd accidentally press `z` instead of `a` and see this mistake in the prompt.

Currently only `<esc>`, `<ret>` and `<backspace> (on an emtpy prompt)` abort the jump.