Subject: bug#35334: 26.2; Changes to position of point are undone during with-current-buffer if window is visible but not current

From: Sean McAfee <[email-masked]>
Date: Sun, 21 Apr 2019 16:32:48 -0700

On Fri, Apr 19, 2019 at 9:10 PM Noam Postavsky <[email-masked]> wrote:

> Sean McAfee <[email-masked]> writes:
> > I would expect point to move to position 8 in both cases, and I don't
> > see anything in the documentation for with-current-buffer or goto-char
> > to suggest that it ever wouldn't.
>
> How about in (elisp) Window Point?
>
>    * Selecting a window sets the value of point in its buffer from the
>      window's value of point.  Conversely, deselecting a window sets the
>      window's value of point from that of the buffer.
>

Okay, I wasn't fully aware of these window-point semantics.  I'll take a
step back and describe my actual problem, of which the bug I thought I was
reporting was the simplest case I could reduce it to.

Starting with two windows visible, the scratch buffer and a window showing
empty buffer "a", I evaluate this expression in the scratch buffer:

(start-process "count" "a" "bash" "-c" "for i in {1..10}; echo $i; sleep 1;
done")

I see the numbers 1-10 inserted into buffer "a", one second apart, as
expected.  The position of point in buffer "a" follows the new lines as
they're inserted.

Now I define the function ordinary-insertion-filter, as found on this page:

https://www.gnu.org/software/emacs/manual/html_node/elisp/Filter-Functions.html

That function is described as "mimicking the actions of the default
filter."  However, if I erase buffer "a" and evaluate this expression in
the scratch buffer:

(let ((proc (start-process "count" "a" "bash" "-c" "for i in {1..10}; echo
$i; sleep 1; done")))
  (set-process-filter proc #'ordinary-insertion-filter))

...then I see the numbers inserted as before, but now the position of point
in buffer "a" doesn't move; it stays at the beginning of the buffer.  If I
select the window showing buffer "a" as the numbers are still being
inserted and move point to the end of the buffer, then point starts
following the new numbers.  But if I select the scratch buffer again, point
in buffer "a" stops following the new numbers.

Is this the expected behavior?