70.0.3538.16 (Official Build) dev (64-bit)
Platform	11021.13.0 (Official Build) dev-channel chell
Firmware Version	Google_Chell.7820.315.2
ARC	5003685

What steps will reproduce the problem?
(1) Enable chrome://flags/#ash-enable-unified-desktop
(2) Attach an external monitor
(3) Lock screen
(4) Detach external monitor

What is the expected result?
The lock screen protects everything behind it.

What happens instead?
The lock screen doesn't cover the task bar, and the chrome, mail, secure shell etc icons can be interacted with using the mouse. (A tool tip is displayed on hover)