bug#29111: 26.0.90; Erc keep-place module broken with new default of switch-to-buffer-preserve-window-point

[martin rudalics <rudalics@gmx.at>]

 > 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