bug#14476: 24.3; mapping newline in buffer-display-table confuses posn-at-point

[Jonas Hörsch]

this bug ultimately leads to hard to understand column jumping in
buffers where whitespace.el is active (and whitespace-style includes
newline-mark). 

a small show case is provided by the following three expressions. the
first two set it up. the third is to demonstrate the bug. tested in
emacs 24.3.1 started with emacs -Q.


(setq buffer-display-table (make-display-table))
(aset buffer-display-table ?\n [?$ ?\n])
(car (posn-x-y (posn-at-point)))


;; C-x C-e with point on the empty line right above this comment will
;; return the column 8 instead of the expected column 0. this leads to
;; seemingly erratic behaviour in some use-cases:
;; entering C-a (beginning-of-line) followed by C-n (next-line) will
;; leave point in column 1 instead of column 0


for any questions or comments feel free to ask away,

jonas


In GNU Emacs 24.3.1 (x86_64-unknown-linux-gnu, GTK+ Version 2.24.14)
 of 2013-05-01 on localhost
Windowing system distributor `The X.Org Foundation', version 11.0.11204000
Configured using:
 `configure '--disable-dependency-tracking'
 '--prefix=/nix/store/jrw95x1nly7xghf5d7kx1xsvl9116b6c-emacs-24.3'
 '--with-x-toolkit=gtk' '--with-xft'
 '--with-crt-dir=/nix/store/zpr8jdx10napidkyz62f3nzl5fm87jyr-glibc-2.17/lib''

Important settings:
  value of $LANG: en_GB.UTF-8
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Emacs-Lisp

Minor modes in effect:
  tooltip-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: (only . t)

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Making completion list...
#^[nil nil display-table nil nil nil nil nil nil nil nil nil ...]
[36 10]
8 [2 times]
Type C-x 1 to delete the help window.
Quit
Saving file /home/coroa/test-case.el...
Wrote /home/coroa/test-case.el

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mail-prsvr mail-utils help-fns disp-table help-mode easymenu time-date
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