Subject: bug#29050: 26.0; Change in when `window-configuration-change-hook' is run

From: Drew Adams <[email-masked]>
Date: Sat, 28 Oct 2017 18:25:35 -0700 (PDT)

This change does not seem right (from NEWS):

 *** Resizing a frame no longer runs 'window-configuration-change-hook'.
 'window-size-change-functions' should be used instead.

Previously you could have a hook on 'window-configuration-change-hook'
that would take effect for frame resizings.  Now you cannot.

Perhaps someone thought that just telling users to use
'window-size-change-functions' instead would suffice.  No.  That hook
(which already existed, and which was fine as it was) is for ABNORMAL
hooks.  This incompatible change means that you cannot use the same,
NORMAL hook for both'window-configuration-change-hook' and
'window-size-change-functions'.

So if you want the behavior you had before, i.e., you want a function to
be invoked for both kinds of changes, you are out of luck.  You need to
have two different functions, or you need to at least change the
function to accept a frame argument, even if it is not used.  Why?

Example:

(define-minor-mode pretty-control-l-mode
    "Toggle pretty display of Control-l (`^L') characters.
With ARG, turn pretty display of `^L' on if and only if ARG is positive."
  :init-value nil :global t :group 'Pretty-Control-L
  (if pretty-control-l-mode
      (add-hook 'window-configuration-change-hook 'refresh-pretty-control-l)
    (remove-hook 'window-configuration-change-hook 'refresh-pretty-control-l))
  (walk-windows
   (lambda (window)
     (let ((display-table  (or (window-display-table window)
                               (make-display-table))))
       (aset display-table ?\014 (and pretty-control-l-mode
                                      (pp^L-^L-display-table-entry window)))
       (set-window-display-table window display-table)))
   'no-minibuf
   'visible))

The hook function no longer kicks in for "frame resizing", which also
means that it no longer kicks in when a frame is created.  So now the
code needs to add the hook function to both hooks (a normal hook and an
abnormal hook).  And the hook function, `refresh-pretty-control-l', now
needs to be changed to accept a phantom FRAME arg:

(defun refresh-pretty-control-l (&optional _)
  "Reinitialize `pretty-control-l-mode', if on, to update the display."
  (interactive)
  (when pretty-control-l-mode (pretty-control-l-mode t)))

Why this incompatible change?

In GNU Emacs 26.0.90 (build 3, x86_64-w64-mingw32)
 of 2017-10-13
Repository revision: 906224eba147bdfc0514090064e8e8f53160f1d4
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''