Resumption after using an external monitor causes strange flashing that shows the _unlocked_ desktop

[LinuxOnTheDesktop]

 * Cinnamon-Screensaver version: 5.2.1
 * Distribution: Mint 20.3 Cinnamon
 * Graphics hardware *and* driver used: Intel; Mesa
 * 32 or 64 bit: 64

Issue

I connected a laptop to an external screen and then disconnected the screen. Then after a while and by design the laptop slept. Upon resume - after I tapped the power button - the desktop showed for a second, then disappeared, and then after some more flickering (showing I'm unsure what) finally the lock screen appeared and everything worked as normal.

EDIT: And the problem persists until a reboot.

Steps to reproduce

1. Connect a laptop to an external screen. (I used a HDMI cable.)
2. Use the external screen. (I did not use mirroring. I did at one point send a window to another workspace, on the mistaken assumption that this would make the window appear on the external screen. Then via Settings -> Display I fiddled with the external display placement. Then I dragged a window onto the external monitor.)
3. Disconnect the external monitor.
4. Wait until the laptop sleeps.
5. Wake the laptop up.

Expected behaviour

Upon awaking the machine, the lock screen should show and nothing else should happen. Certainly there should be no glimpse of the unlocked desktop.

Other information

This is a security problem or at least a privacy problem. For, my desktop was visible.

Dmesg log attached.
dmesg.txt

[robertbyers1111]

I am able to reproduce this issue in Linux Mint 21 with cinnamon. My cinnamon-screensaver version is 5.4.4.

My procedure to reproduce it is slightly different, but not significantly....

• laptop is running with its own screen and an external monitor, both configured as one extended desktop.
• I manually lock the screen (Desktop menu > Lock screen)
• The screensaver kicks in
• After 30 seconds I turn off power to the external monitor.
• After another 30 seconds I restore power to the external monitor.
• The monitor suddenly displays whatever was on its desktop display at the time I locked the screen.
• It seems to be a read-only display - as none of the windows respond to mouse or keyboard activity, and you only get that single screen display.
• However, even though it is 'only' one screen, it is a serious privacy issue

I am finding various screensaver errors written to /var/log/syslog and /var/log/auth.log

Attachments: 1) my system environment 2) auth.log 3) syslog

environment.txt

auth.log

syslog.log

In addition, there is a thread on forums.linuxmint.com that led us to this defect. That thread is here: Lockscreen security issue

[mtwebster]

Hi please have a look here: #416 (comment)

[LinuxOnTheDesktop]

With the new packages, I have not experienced the flashes and the momentarily unlocked desktop. So perhaps this issue can be closed.

However: I find the following. When using an external monitor, and using Impress to show a slide-show on it, periodically - I would have to investigate the exact conditions - the external monitor stops showing my slides and starts showing my desktop background; in order to fix this, it proves necessary and sufficient to disconnect and then reconnect the external monitor. This might be a problem with Impress. It might be a problem with Cinnamon. I'd appreciate any advice. I have not seen the problem with the older Cinnamon packages.