Just to be clear we are talking about this dialog? There’s no need to double-click exactly on the line, just on the fade zone (above the line). Alternatively if you select the event there’s the Audio > Open Fade Editor(s) option, which opens both the Fade-in and Fade-out dialogs instead of just the one you double-clicked on.
Thank you. Didn’t know that. Someone on YT showed it by clicking on the line a few years ago and that’s how I’ve been using it since. One project doesn’t open the dialogue at all for some reason, so menu alternative is helpful. Thanks again.