ctrl + alt + S on ubuntu 18.04 triggers a screen size toggle but is not listed in keyboard settings

I have found that since Ubuntu 16.04 Keyboard Bindings mostly doesn't show the windows management shortcuts (though occasionally it does, and I've never worked out what controls it).

However, I can check and modify them with dconf or the GUI version dconf-editor. Using this I see that ['<Control><Alt>s'] (lower case) is assigned to /org/gnome/desktop/wm/keybindings/toggle-shaded: this is not the default entry, though I didn't set it explicitly.

You can change the entry to a different shortcut (eg ['<Shift><Control><Alt>s'] or restore the default setting by deleting it completely ([]).

If this isn't the entry in your case, then you can use gsettings to find other locations of the shortcut:-

gsettings list-recursively | grep -i \>s\'

This will find all shortcuts using the s key (plus a few spurious entries, perhaps - note that you can't be sure of the ordering of <Control>, <Shift> and <Alt> within the setting).

Reading @AFH's correct answer, I could not understand just what do do, so here:

gsettings reset org.gnome.desktop.wm.keybindings toggle-shaded

This will turn off this setting.

A few years later, in 2024, Ubuntu 23.10, and the issue is still there ... (I'm trying to access IntelliJ's settings with Control+Alt+S).

At least here gsettings list-recursively | grep -i \>s\' won't list anything.

I'm assuming because the updated Gnome Shell extension screenshot window sizer has it hard-coded. But disabling the extension solves the issue. I'm not sure why it was installed -- maybe I installed it years ago and forgot.

Just in case I created an issue in Gnome Shell Extensions package.