A method, system and computer readable program on a medium for masking a window or a visible part of a window during a copy screen or copy window operation which: provides a primitive to indicate to a window manager whether or not the window comprises information to be copy protected. defines a new window attribute to indicate non-copyability of the window. updates window attributes when this window is copy protected: masks all or part of the window content when the window is no longer active, so that protected information on this window is no longer visible, and displays the original window content when the window becomes active again.