In the Extensions Reference, under Client Query Parameters there is a list of parameters with name, type, and description. As these are query parameters from the URL, a data type doesn’t really make sense, as they are all strings. Specifically, the popout parameter is declared as boolean, yet if I’d parse the parameter out of the url and check whether it === true (or === false respectively) I’d get an unexpected result as the condition would never hold - they are actually the strings true and false when parsed from the URL.
I think it would be better to either omit the data type here to avoid confusion, or to explicitely mark popout (and other future “booleans”) as strings (with the values "true" and "false").