Expose initial URL query parameters to WebApp sessions

Let a Web application read the query string from the browser URL that opened its logical session. Make the value available during App.Opening and page opening so an app can handle links such as /?document=42, referral codes, and repeated parameters without implementing an HTTP endpoint.

Expose the initial query string and convenient lookups for one or all decoded values of a named parameter. Keep these values stable for the session, including across a connection loss and reconnect.