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.
This is a known Xorg issue. Distros like TAILS have patches (can't find a source right now, but it was probably 6+ years ago). The solution is Wayland, since despite TAILS fixing it, no one else seems to have bothered.
(I have the same problem on Fedora 40 XFCE)
KDE did bother, this does neither happen with KScreenlocker, nor do non-screenlocker windows show in another way, because the screen locker is integrated with the compositor.
If the compositor crashes or gets disabled somehow ofc though, that integration doesn't help either and you have to rely on a mountain of bad hacks as well as the hope that the screen locker doesn't also crash for nothing to happen in that case, but it's as close to secure screen locking as you get on Xorg... in the end the solution for secure screen locking is still Wayland.
When my KDE screenlocker crashes on Wayland, all my monitors tell me to switch to a TTY and run a manual unlock. Which is reassuring!