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Window is the easiest implementation of xID, since it requires minimal interference on your website. It also works well on mobile devices. However, remember that the dialogue window might fall behind the browser window on the users desktop, and its easy to loose track of the xID process for the user. A window implementation might initiate popup blockers depending on individual browser settings, and especially when using the login hint XID:unsolicited. Note this special use case for in-app browsers when selecting a window implementation.
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