Revalidation on "keyup" erroneously runs when initially "tabbing" into the field

[Manuel Strehl]

If you use the tab key to switch between form elements, the keyup event is fired on the input where you land at.

This means, our "keyup" event handler will run on any element where you tab to, even if no other interaction took place before.

Fix: Tell the event handler to ignore events with tab keycode.