Control+Alt+S shortcut is used for toggling shaded state in MATE desktop environment. Once toggled, a window's content is hidden except for the title bar. It's useful for peeking at the content behind a window. I personally never use it as I usually just Alt+Tab or place two windows side by side. I prefer to disable it so I can use the 'Save As' shortcut in Notepad++.
Look for `Toggle shaded state` in Keyboard Shortcuts. Disable the shortcut by clicking on the `Ctrl+Alt+S` and backspace.
## Method 2
If you use Compiz, the shortcut is not shown in Keyboard Shortcuts due to conflict with CSSM. You need to manually disable it by editing a dconf value. dconf value can be easily edited through dconf-editor which you can install through `apt` or any other package managers.
Launch dconf-editor and search (click on the search icon on top right corner or just Ctrl+F) for 'shaded'.