When you have a parallel section,
and one of the parallel stages fails,
and you have fail fast set,
All the other parallel stages (that haven't completed) show a message "sending interrupt ..." in the log when they are killed.
It would be nice if the visualization let it be easy to see which of the parallel stages died on their own and which were killed because a different on died. Let's say the little circle turns grey. Then you could easily see which one has the log you need to look at to see why it failed.
I suspect there is some complexity if two stages fail around the same time and then tell each other to die. Maybe this case can't easily be handled. Even if not, it is my opinion that solving the case where only one failed on its own would be useful. Maybe its as simple as putting that grey circle up if the log for that parallel stage has a message about "sending interrupt..." though I doubt that's the real indicator. (If there were two to fail at the same time there would be too many grey circles.)