Yeah, I feel your "pain" when you say you were torn. I also wasn't sure which approach is the better and kept jumping between "let's just omit it" and "let's add a new parameter so people can control this behaviour!"

My internal argumentation as for why I went with option a) (omitting) was that I had a hard time seeing actual value in duplicating the output. And based on that I thought not keeping it around as a default case would benefit more users than adding a customisation option that nobody would even see 😅

I almost think I'd like this output to be truncated to some length ...

Yeah, I guess that makes sense, especially if your output isn't as bad as mine.
Do you also have a good enough guesstimate about a reasonable length you'd like to see the output being trimmed to? ;)
I think what could make sense here is keeping the last, let's say 1000 characters, assuming that errors likely are printed at the end?
For the record: in my use-case (= Unity) keeping the last 1k characters wouldn't help, as Unity tends to output a lot of spam after errors too, before exiting 😄

and then maybe have the full output stored somewhere so that it can be seen a bit easier? 🤔

That is more or less available, right? Once you pass a callback, of course.
See https://github.com/fastlane/fastlane/blob/master/fastlane/lib/fastlane/helper/sh_helper.rb#L78

Another thought I had while looking at it, but got afraid as that feels like a bigger change, is that I could change from popen2e to some other method that isn't merging stdout and stderr of the executed process.
Which would mean that I would have the possibility to print stderr only in the error case, and not having to repeat the entire stdout again.

What do you think?

Friendly reminder to @joshdholtz, if you have some time please don't forget this small PR ;)

Also, warm welcome to the newcomer 👶 👋

@tutipeti Ahhhh, I missed your response! Thank you for the friendly ping ❤️

@joshdholtz I didn't really have a better idea so ended up placing back the repeated output, but shortened it to the last 500 characters.

If you have another wish please do tell me :) and happy new year 😅

Hey @joshdholtz, fyi I'm still waiting for an approval here. And the fastlane bot keeps trying to close the issue related to this change request ;)

Any chance we can get this approved?