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Inan option was added to user profiles to allow individual users to disable the Overlay for their account. This option requires the user to be authenticated. There is no option for an unauthenticated user to disable Overlay.
Although it is unlikely that an unauthenticated user will encounter Overlay it is possible for a D7 site to be configured in a way that makes this possible, therefore we need a mechanism to allow unauthenticated users to disable the Overlay for their session.