Comment 61 for bug 1033226

Matthew Brush, your patch is a first step, though re-adding the close button would just increase inconsistency between Compiz/Unity 7 (where dialogs can have close buttons) and Unity 8 (where none will).

More importantly, in its current state the patch contains a security vulnerability: if someone chooses "Restart Later" the system is left in an insecure state indefinitely, dependent on the user remembering that they had been told -- hours, days, or weeks ago -- that they needed to restart. Even if they reopened Software Updater manually it wouldn't tell them. That's why I outlined five steps to fixing this bug, of which your patch is #2.