Screen unlocks when external monitor connected to laptop

[Jan Badkamer]

Lock screen is completely bypassed simply by connecting external HDMI screen to laptop.

To reproduce:
1. Connect external monitor, with laptop display disabled
2. Lock screen
3. Verify screen locked by moving trackpad, confirm that password prompt is displayed
4. Disconnect external monitor
5. Verify (laptop) screen locked by moving trackpad, confirm that password prompt is displayed
6. Re-connect external monitor
7. Working, *unlocked* system is displayed on external monitor, no password prompt at all

Ubuntu Mate 18.04
Kernel: Linux 4.15.0-22-generic
mate-screensaver 1.20.0
lightdm 1.26.0

[Victor Kareh (vkareh)]

I was able to reproduce this finally.

1. Put laptop on suspend mode (close the lid, etc)
2. Open the laptop lid and confirm there is a password prompt
3. Now put the laptop back on suspend mode
4. Connect an HDMI monitor
5. Open the laptop lid
6. Sometimes the password prompt is displayed, but unusable (no way to focus on input field). Other times the desktop is displayed and the laptop is entirely usable.

Note: I restarted marco when on this "unauthorized" mode, and it then displayed the password prompt with proper focus.

[Mr Tonik (mr-tonik)]

Can reproduce it in two ways:

1. Connect external monitor, with laptop display disabled.
2. Suspend the system
3. Detach external monitor
3. Resume the system
4. No lock screen displayed. Full access to regular desktop session.
5. Laptop screen cant be locked again, unless you kill mate-screensave.

1. Suspend the laptop
2. Connect an HDMI monitor
3. Resume -> External monitor gets enabled, laptop screen disabled.
4. No lock screen displayed. Full access to regular desktop session.
5. External Monitor cant be locked again, unless you kill mate-screensave.

If I dont kill mate-screensave and restore the monitor configuration to the state it was when the system was suspended, by connecting or removing the external monitor, the lock screen gets displayed again immediately.
Looks like only the display that is active in the moment of the actual locking is getting locked.
Instead of just locking every display, even if that may changes, and displaying the lock screen accordingly.

Also looks like related to https://bugs.launchpad.net/ubuntu-mate/+bug/1777889