Ubuntu 16.04 Ctrl+Alt+S doesn't work
It appears that the window manager already captures the key combination Ctrl+Alt+S.
To unbind it or rebind it to a different key combination
- open the Control Center,
- navigate to Keyboard → Shortcuts,
- select the Windows shortcuts,
- look for the Toggle shaded state shortcut entry and
- edit it to a different or no key combination.
This applies to at least Unity and GNOME since they both use the same settings entries to determine keyboard shortcut combinations.
It's possible that the key combination is not bound to the same shortcut as in OP's case, in which case you need to look through all shortcut entries for one that has the key combination in question.