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You should not be editing posts for subjective stylistic changes based on the personal coding preferences of the coder. If a code sample is completely unreadable (i.e. not formatted as code, has no whitespace, etc.) then applying some basic formatting to make it generally readable is okay. Taking perfectly readable code and altering it to use your personal preferences is not. Allowing such edits would just result in edit wars between people with conflicting preferences and would waste a lot of people's time without actually improving the post at all.
As for which reason to choose, I'd probably go with "invalid edit". "too minor" is probably technically correct as well, but it would probably be more confusing. The edit isn't a minor change in terms of how much is changed, it's minor in terms of the value added, but that's less obvious to the reader. This is also a case where I'm somewhat inclined to break out the custom reason to say that edits shouldn't be used to make changes based on the personal style preferences of the editor, despite my hesitation to use custom reasons in general.
I typically reject as invalid edit. Some people are really bad about it, though in that case, he has stopped doing it.
Now, that said, I do change code blocks like this:
To something with no horizontal scrollbars:
But I do this simply so I can read the bleeping question, not so that it appears in my preferred style or so that I get rep or badges or what have you. I don't do this as a suggested edit and it doesn't cause anyone else any work. If I didn't have enough rep to make the edit myself, and if I at least had any clue that a suggested edit causes other people work, I probably wouldn't suggest it unless it was really bad, like this: