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bug#35334: 26.2; Changes to position of point are undone during with-cur


From: Sean McAfee
Subject: bug#35334: 26.2; Changes to position of point are undone during with-current-buffer if window is visible but not current
Date: Sun, 21 Apr 2019 16:32:48 -0700

On Fri, Apr 19, 2019 at 9:10 PM Noam Postavsky <address@hidden> wrote:
Sean McAfee <address@hidden> 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?


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