Ubuntu – How to prevent the screen from locking when I want a virtual machine to ungrab the mouse

You can remap the key using ~/.Xmodmap. Use xev (in a terminal) to find the offending keycode, then assign a different keysym, e.g. XF86WakeUp. Append this line to ~/.Xmodmap (create that file if it does not yet exist):

keycode 150 = XF86WakeUp

After logging out and logging in again your key should be mapped. If you don't want to log out, you can also use xmodmap ~/.Xmodmap to reload the key mappings.

Using xev to actually find the keycode of the key that you want to remap might be not so easy: if the key is already bound to an action (e.g. in the Gnome keyboard properties), xev does not show the keycode. As an example, this is the kind of xev output that you want to see:

KeyPress event, serial 41, synthetic NO, window 0x5800001,
    root 0xb3, subw 0x0, time 312883, (737,182), root:(946,647),
    state 0x0, keycode 180 (keysym 0x1008ff18, XF86HomePage), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

You can see the keycode (180) in the third line after the word "keycode". If you don't get such a block that starts with "KeyPress event", your key probably is already bound. You can try to use a non-Gnome desktop like LXDE or KDE and use xev there.

Unfortunately, the lock-screen utility gnome-screensaver-command, which the Ctrl+Alt+L shortcut uses, will also activate the blank-screen "screensaver" whenever it is told to lock the screen.

The trick then is to "fake" a keystroke immediately after the screen is locked to get rid of the blank-screen and leave the lock-screen on. We'll do this with xdotool, a small command-line utility which allows automation of keyboard and mouse clicks/movements.

1. Install xdotool with

   sudo apt-get install xdotool
   

2. Paste the following to a file in your home directory, such as locknoblank.sh:

   #!/bin/bash
   
   gnome-screensaver-command -l
   xdotool sleep 1 key Ctrl
   

   • This simply locks the screen normally, but one second later, sends a dummy Ctrl keystroke to "unblank" the screen.

   • Make the file executable with

     chmod +x ~/locknoblank.sh
     

3. Go to System Settings...Keyboard...Shortcuts and remove Ctrl+Alt+L as the lock-screen shortcut so we can re-use it for our custom command. Click on "System" on the left sidebar, then click on the right of "Lock Screen" where it says "Ctrl+Alt+L"; it will now say "New Accelerator...". Press Backspace to remove the shortcut.

   

4. Now go to Custom Shortcuts on the left, and add a shortcut, naming it whatever you want to with the command as /home/username/locknoblank.sh. Click Apply, then click on its right where it says "Disabled" to get "New Accelerator", and press Ctrl+Alt+L.

   

Close System Settings, and you're done!

To prevent reactivation of the screensaver (idle timeout)

Open a terminal, and type/paste:

gsettings set org.gnome.desktop.session idle-delay 7200

This sets the screensaver idle activation time to 2 hours (or 7200 seconds). You can set it to anything you want; the default is 600.

Note: For Ubuntu 18.04, use

xdg-screensaver lock

instead of gnome-screensaver-command -l.