Why mark gnus-summary-buffer as the previous buffer when opening say a pdf attachment?

[Yuchen Pei]

Referring to the following lines in `mm-display-external':

--8<---------------cut here---------------start------------->8---
                          ;; So that we pop back to the right place, sort of.
                          (switch-to-buffer gnus-summary-buffer)
                          (switch-to-buffer mm))
--8<---------------cut here---------------end--------------->8---

I don't know how to open an attachment from the summary buffer, or
whether this is the representative usecase of opening a attachment.

But if the attachment is opened from an article buffer (e.g. an email
message), the effect of these lines is that it pops back to the "wrong"
place (list of emails) when one is done with viewing
(e.g. `quit-window').

So I wonder what is the rationale of this logic.

Best,
Yuchen

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