Can start PIE in New Editor Window during PIE instance's online login

When starting a PIE instance using the "Selected Viewport" play mode, the ability to start another PIE session is blocked (handled in FInternalPlayWorldCommandCallbacks::PlayInViewport_CanExecute). This check is not present in FInternalPlayWorldCommandCallbacks::PlayInEditorFloating_CanExecute, which allows the user to try to continually start PIE sessions, even if an earlier one started some async task (such as logging into an online service) that it is waiting to complete.

With PIE logins enabled (Editor Preferences -> Level Editor -> Play Credentials -> Enable Logins), start a PIE instance using the "New Editor Window (PIE)" play mode. While this instance is logging in to the online service (before OnLoginPIEComplete_Deferred has been called), it is possible to press the editor's Play button again.