I have an older Intel laptop that has a 1600x900 display, and I find that if I put the machine to sleep, connect an external monitor with a higher resolution, and then turn it back on, the login screen doesn't adjust to the new resolution and it reveals what I had open (see photo).
However, I'm not that familiar with Linux Mint (even though I've daily driven Linux for nearly 10 years, I very casually use LMDE) and I'm not sure if this is a Cinnamon problem or if the lock screen is under a different program.
Looking at Linux Mint's webpage on reporting a bug (https://projects.linuxmint.com/reporting-an-issue.html) they seem to mostly use Cinnamon as an example, but I don't want to report this issue as a Cinnamon issue if it's the wrong project.
In case this is platform specific, my device's details are below:
- Host: Dell Latitude E6420
- CPU: Intel Core i7-2630QM (Sandy Bridge)
- GPU: Intel 2nd Generation Core Processor Family
- Kernel: 6.1.0-21-amd64
- DE: Cinnamon 6.0.4
- WM: Mutter (Muffin)
- Display Server: X11
I've never filed a bug report in my life before, usually I just put up with the issue until it's eventually fixed, but I feel this is a moderate security issue that should be flagged.
That definitely looks like a Cinnamon bug, specifically its screensaver component. Worst case they'll direct you to where to report the bug, or they'll move the ticket themselves.
Also, obligatory this is one of the many ways X11 is insecure and unsuited to proper screen locking. Lock screens on X11 are just fullscreen windows you pray the window manager won't ever allow to unfocus, close, resize or move or not cover the rest of the desktop.