On 2016-06-24 15:13, Eli Zaretskii wrote:
> What is the purpose of calling set-window-buffer?

When started, Proof General splits the screen in two:

|             |             |
|             |             |
| User's file | Message log |
|             |             |
|             |             |

Each time a message is received, it's displayed in the Message log window.
My extension sometimes reuses that window for other things (for example, to show company-mode's documentation buffer).
The set-window-buffer call is there to ensure that the user sees new messages. Of course in most cases that call is useless: that's why predicating it on the window not already displaying the message log makes things faster.

> Also, how come it doesn't happen on your machine?  Are any
> customizations involved?  Is your OS different from that of the other
> user?

Somewhat unclear. The OS are the same (Linux mint), and the problem does happen in emacs -Q for the other user. The version of Emacs is the same.