I agree, this would also prevent from removing the subject when the user keeps "backspace" pressed down.

Though I'm not sure using keyup will prevent this. Ideally: keeping backspace pressed when there is some text should not remove the subject line, but when the user starts pressing the backspace and there is no content, then keyup will remove the subject line.

So we can do this:
* keydown: if this is backspace, and if there is some content, set some state "emptyAtKeydown", otherwise remove the state (to prevent cases where we have keydown but no keyup, could possibly happen if the user moves the finger out of the screen)
* keyup: if this is backspace and `emptyAtKeydown` is true, set `emptyAtKeydown` to false and remove the subject line.

Does this sound good?