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Disable Ctrl-Alt-L's behavior of locking the screen
February 19, 2018
Edited: June 12, 2020
November 24, 2022
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Disable nothing and use
meta key (aka Super or Windows key)
Hold down Ctrl+Alt+Meta+L and Ubuntu will ignore this for Lock Screen but your application will interpret it as Ctrl+Alt+L.
Disable Ctrl+Alt+T in Ubuntu shortcuts
Go to System Settings -> Keyboard -> Shortcuts -> System -> Lock Screen and hold down Ctrl+Alt+Shift+Z
This will hopefully allow you to reassign the original key combination in another application.
To disable lock screen permanently
gsettings set org.gnome.desktop.lockdown disable-lock-screen 'true'
On my dual monitor system the lock screen no longer appears when pressing Ctrl+Alt+L. However the screens saver still kicks in and you have to move mouse, touch a key, etc to turn the monitors back on. Although a password is not required to unlock the screen it might be a bug.