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It sounds like you're encountering a kernel-level or X/Wayland-level freeze while developing. The fact that the keyboard is unresponsive except for REISUB suggests a deep hang in the graphical session or related system processes.

Here’s a practical way to debug and diagnose this issue with two displays and one host, without needing a network:

Setup for Debugging on Two Displays

1. Enable Persistent Kernel Logs on Second Display:

   • Use a TTY session on your second display to monitor kernel logs.
   • Connect your second display and switch to it using Ctrl+Alt+F2 (or F3-F6, depending on availability).
   • Login and run:

     sudo tail -f /var/log/kern.log
     

     or

     sudo dmesg -w
     

   • This will show you kernel logs in real time, which is often critical for diagnosing freezes.

2. Mirror Logs or Virtual Console Across Displays:

   • If your second display doesn't switch TTYs independently, use a terminal multiplexer (like tmux or screen) to duplicate the kernel log view.
     • Start tmux in TTY1 and attach to it from TTY2:

       tmux
       tail -f /var/log/kern.log
       

       Then on the second TTY:

       tmux attach
       

Improve Debugging Workflow

3. Set Up Magic SysRq for More Control:

   • If your system allows REISUB, the Magic SysRq key is already enabled. You can use other commands to debug live:
     • Sync and Kill Tasks: Alt + SysRq + F can attempt to kill resource-hogging processes.
     • Toggle TTY: Alt + SysRq + R can re-enable raw keyboard input if the GUI is frozen.

4. Enable Kernel Debugging Outputs:

   • Append debug to your kernel boot parameters in GRUB:
     • Edit /etc/default/grub:

       GRUB_CMDLINE_LINUX_DEFAULT="quiet splash debug"
       

     • Update GRUB and reboot:

       sudo update-grub
       

   • This provides richer debugging output in logs.

5. Use a Non-GUI Target:

   • For safer development, switch to a lower system target to prevent full GUI freezes:

     sudo systemctl set-default multi-user.target
     

     • Reboot into a non-GUI mode where you can manually start your app, making it easier to debug without freezing the entire desktop.

6. Forcefully Isolate the Process:

   • Run your wallpaper application in a separate X server to isolate it:

     xinit /path/to/your/app -- :1
     

     • This starts a new X session on display :1 while keeping your primary desktop untouched.

Tips to Prevent GUI Freezes During Development

• Limit CPU or Memory Usage:
  • Use ulimit to restrict resource usage of your program:

    ulimit -v 1048576  # Limit to 1GB virtual memory
    

• Log Debugging Information:
  • Add debug prints in your app and log them to a file instead of stdout.