Comment 4 for bug 47643

We came across a similar issue, might be the same.

Our installer (di-live) uses debconf to interact with the user, which in turn uses the available debconf frontend, which happens to be whiptail (ie. newt).

When the key combination ALT + *anykey* (not only tab) is pressed, whiptail will exit with code 255.
This is easily reproducable at the prompt:

    $ whiptail --yesno test 20 50; echo $?
    press ALT + *anykey*

BTW - the same issue is present in dialog, so the problem is probably not in newt itself...