Screen-flash loop makes sign-in impossible

The July preview update introduced a repeating screen-flash loop at the Windows sign-in screen. The display would flash continuously, rendering the system completely inaccessible through normal means. Users couldn’t type their password, couldn’t click anything, couldn’t do anything but watch their screen strobe. The only escape was Ctrl+Alt+Delete to reach Task Manager — assuming you could time the keyboard shortcut between flashes. A sign-in screen that actively prevents signing in is a bold design choice.