martin rudalics <rudalics@gmx.at> writes:

>> Keep place adds a hook so that, when a new message comes in on a
>> non-visible buffer *and* point is at the bottom, point is moved up by
>> one line. Doing this means that point stays on the last 'read'
>> message. Here's the relevant code from keep-place:
>>
>>> (defun erc-keep-place (ignored)
>>> "Move point away from the last line in a non-selected ERC buffer."
>>> (when (and (not (eq (window-buffer (selected-window))
>>>                      (current-buffer)))
>>>             (>= (point) erc-insert-marker))
>>>    (deactivate-mark)
>>>    (goto-char (erc-beg-of-input-line))
>>>    (forward-line -1)))
>
> You could try to pretend that the position at which the buffer was
> previously shown in a window is the position calculated here.  Add after
> the
>
> (forward-line -1)
>
> something like
>
> ;; For all non-minibuffer windows on all frames check whether the
> ;; current buffer was shown in that window before.  If so, update the
> ;; window point positions stored for the buffer to the position we just
> ;; calculated.
> (dolist (frame (frame-list))
>   (walk-window-tree
>    (lambda (window)
>      (let ((prev (assq (current-buffer) (window-prev-buffers window)))
> 	   (next (assq (current-buffer) (window-next-buffers window))))
>        (when prev
> 	 (setf (nth 2 prev) (point-marker)))
>        (when next
> 	 (setf (nth 2 next) (point-marker)))))
>    frame nil 'nominibuf))
>
> Completely _untested_ so you may have to tweak it appropriately!
>
>> Previously, this worked fine, since moving point on non-visible buffers
>> meant the movement persisted till the buffer was next 'shown'. However,
>> the new default of `switch-to-buffer-preserve-window-point' ensures that
>> point stays wherever it was 'last seen'. I'm not sure how it does it,
>> but I think it's saving point when a buffer is hidden and restoring it
>> when it's seen again
>>
>>> (when (and entry
>>> 		   (or (eq switch-to-buffer-preserve-window-point t)
>>> 		       displayed))
>>> 	  ;; Try to restore start and point of buffer in the selected
>>> 	  ;; window (Bug#4041).
>>> 	  (set-window-start (selected-window) (nth 1 entry) t)
>>> 	  (set-window-point nil (nth 2 entry)))
>
> Right.  So with the hack above you would set (nth 2 entry) to the value
> keep-place calculated.
>
>>> That's a possibility, but it would be too radical to go into 26.1, so
>>> I think we should explore the less drastic alternatives first.
>>
>> I agree with Eli that this is too big of a change at this point, but I
>> think this is the best long term solution. Perhaps we could
>> `make-variable-buffer-local' on `switch-to-buffer-preserve-window-point'
>> when enabling keep-place (later, of course).
>
> I can't yet assess all implications of such a solution so I'd certainly
> defer it until all other measures have been exhausted.
>
>>> Since ‘keep-place’ is some sort of a minor mode, enabling it should warn
>>> the user about the ‘switch-to-buffer-preserve-window-point’
>>> incompatibility.  But I'm not familiar with ‘define-erc-module’ so we'd
>>> probably need someone knowledgeable to do that.
>>
>> I think this is probably the best idea before the emacs 26 release. The
>> definition of 'keep-place' is currently
>>
>>> (define-erc-module keep-place nil
>>>    "Leave point above un-viewed text in other channels."
>>>    ((add-hook 'erc-insert-pre-hook  'erc-keep-place))
>>>    ((remove-hook 'erc-insert-pre-hook  'erc-keep-place)))
>>
>> I think we should be able to simply do something like
>>
>>> (define-erc-module keep-place nil
>>>    "Leave point above un-viewed text in other channels."
>>>    ((add-hook 'erc-insert-pre-hook  'erc-keep-place)
>>>     ;; poor name, but just an example
>>>     (erc-check-if-switch-to-buffer-preserve-and-warn))
>>>    ((remove-hook 'erc-insert-pre-hook  'erc-keep-place)))
>>
>> I would be happy to submit a patch for this, if the Erc maintainer is
>> busy. Does this seem like a good solution, or does anyone see a better
>> way around this?
>
> If all else fails let's do that.  First please try the proposal above.
>
> martin