People complain about copying a post's text into the clipboard and pasting it elsewhere causes every new line to be indendet by one space.

Quoting my own post:

there's a line feed character after each <br>, which is then "converted" by many internet browsers to a whitespace. That's also the reason why an empty line translates to one space character. If phpBB would not send said line feed then pasting would be no problem either.

If it would be a problem of the internet browsers only then why would pasting said text to other programs (i.e. Window's Notepad) give me the same "weird" results?

I can assure you phpBB3.0.x did this not - it only converted any "\n" purely to "<br/>", not "<br/>\n".

This can be reproduced by copying post text or a preview text. Doesn't matter where you paste it.