Every so often when I’m using the start menu I’ll bump the button that undocks my machine, drives me nuts. This is how you turn the undock feature on and off if that happens to you:
Type gpedit.msc in the Start menu’s search box and then press Enter.
Navigate to User Configuration, Administrative Templates, and then select Start Menu and Taskbar in the left column of the Group Policy editor.
Double-click Remove the “Undock PC” button from the Start Menu in the Settings section of the Group Policy editor.
Select Enable and then click OK to save the changes.