Have you thought about changing the behavior of the reply window?
Right now, if I want to quote part of a response in this format:
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> here_is_the_quoted_message that I am copying from the person I'm talking to
[ here I am replying to the person ]
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And then when I want to select a part of the text from my interlocutor again, I get this:
It would be more convenient if, while the reply window is open, the user could freely select and copy the text of the post they are currently replying to.