It's confusing that when showInputBox is given an invalid value that pressing <enter> just closes the box and returns undefined. I've had users that have said things are broken because they typed a value and pressed enter - their value was invalid and they didn't notice the prompt (because they typed and pressed enter quickly).

Either we should be able to tell when the user pressed <enter> on an invalid value, or they shouldn't be able to - the box should just stay visible with the validation error until the fix it or use <esc> to cancel.