Emacs 21.2 display bugs: variables: cursor-type; properties: display, before-string, after-string, invisible, intangible

[Joe Wells <jbw@izanami.cee.hw.ac.uk>]

;; Emacs 21.2 display bugs
;; affected variables: cursor-type
;; affected overlay properties: display, before-string, after-string, invisible, intangible

;; bugs reported by: Joe Wells (<URL:http://www.cee.hw.ac.uk/~jbw/>)

;; In GNU Emacs 21.2.1 (i386-redhat-linux-gnu, X toolkit, Xaw3d scroll bars)
;;  of 2002-05-07 on stripples.devel.redhat.com
;; configured using `configure  i386-redhat-linux --prefix=/usr --exec-prefix=/usr --bindir=/usr/bin --sbindir=/usr/sbin --sysconfdir=/etc --datadir=/usr/share --includedir=/usr/include --libdir=/usr/lib --libexecdir=/usr/libexec --localstatedir=/var --sharedstatedir=/usr/com --mandir=/usr/share/man --infodir=/usr/share/info --with-gcc --with-pop --with-sound'
;; Important settings:
;;   value of $LC_ALL: nil
;;   value of $LC_COLLATE: nil
;;   value of $LC_CTYPE: nil
;;   value of $LC_MESSAGES: nil
;;   value of $LC_MONETARY: nil
;;   value of $LC_NUMERIC: nil
;;   value of $LC_TIME: nil
;;   value of $LANG: C
;;   locale-coding-system: nil
;;   default-enable-multibyte-characters: t

;; Test cases to reproduce the buggy behavior are supplied for all
;; bugs reported in this message.

;; * The buffer's value of cursor-type is ignored when the buffer's
;; window is not the selected window.  Instead, the usual hollow box
;; cursor is displayed.

(progn
  (delete-other-windows)
  (kill-buffer (get-buffer-create "test"))
  (set-buffer (get-buffer-create "test"))
  (setq cursor-type 'bar)
  (display-buffer (current-buffer)))

;; * A display property of "" (the empty string) on any character in
;; the buffer causes a truly wacky event.  The buffer is effectively
;; partitioned into two portions, that before the first such
;; character, and that after.  Only one portion will be displayed at a
;; given time, generally the portion that point is in.  Furthermore,
;; subsequent display property values appear to be ignored.  There are
;; 3 test cases for this bug, displaying various aspects of it.  The
;; buggy behavior is affected by the value of scroll-step; if
;; scroll-step has its default value of 0 then the behavior is even
;; worse than I have described.

(progn
  (kill-buffer (get-buffer-create "test"))
  (set-buffer (get-buffer-create "test"))
  (insert "ABCDEFGH")
  (let ((o (make-overlay 3 4))) ;; covering the "C"
    (overlay-put o 'display ""))
  (let ((o (make-overlay 7 8))) ;; covering the "F"
    (overlay-put o 'display ""))
  (display-buffer (current-buffer)))

(progn
  (kill-buffer (get-buffer-create "test"))
  (set-buffer (get-buffer-create "test"))
  (insert "ABCDEFGH")
  (let ((o (make-overlay 3 4))) ;; covering the "C"
    (overlay-put o 'display ""))
  (let ((o (make-overlay 7 8))) ;; covering the "F"
    (overlay-put o 'display ""))
  (goto-char (point-min))
  (display-buffer (current-buffer)))

(progn
  (kill-buffer (get-buffer-create "test"))
  (set-buffer (get-buffer-create "test"))
  (insert "ABCDEFGH")
  (let ((o (make-overlay 3 4))) ;; covering the "C"
    (overlay-put o 'display ""))
  (let ((o (make-overlay 7 8))) ;; covering the "F"
    (overlay-put o 'display "Z"))
  (display-buffer (current-buffer)))

;; * The "specified spaces" form of display specification causes wacky
;; cursor problems when the space is very small.  When point is just
;; before the first character of a range having such a specification,
;; the cursor is not displayed when the buffer's window is selected.
;; It does not matter whether the cursor is the default box cursor or
;; the bar cursor.  When the buffer's window is not selected, the
;; cursor is displayed as a hollow box that extends all the way to the
;; right edge of the window.  There are various other display things
;; that go wrong in this situation, too many to list.

(progn
  (kill-buffer (get-buffer-create "test"))
  (set-buffer (get-buffer-create "test"))
  (insert "ABCD")
  (goto-char 2) ;; between A and B
  (let ((o (make-overlay 2 4))) ;; covering the "BC"
    (overlay-put o 'display '(space :width 0.01)))
  (display-buffer (current-buffer)))

;; * A string-valued display property on a range of characters causes
;; cursor problems.  If point is just before all of the characters
;; with the string-valued display property, the cursor is displayed
;; _after_ the string which is displayed instead of the characters.

(progn
  (kill-buffer (get-buffer-create "test"))
  (set-buffer (get-buffer-create "test"))
  (insert "ABC")
  (let ((o (make-overlay 2 3))) ;; covering the B
    (overlay-put o 'display "Y"))
  (goto-char 2) ;; between A and B
  (display-buffer (current-buffer))
  (what-cursor-position) ;; shows point is _before_ the B
  )

;; * A before-string property on a range of characters causes cursor
;; problems.  If point is just before all of the characters with the
;; before-string property, the cursor is displayed _after_ the string
;; which is the value of the before-string property.

(progn
  (kill-buffer (get-buffer-create "test"))
  (set-buffer (get-buffer-create "test"))
  (insert "AB")
  (let ((o (make-overlay 2 3))) ;; covering the B
    (overlay-put o 'before-string "Y"))
  (goto-char 2) ;; between A and B
  (display-buffer (current-buffer))
  (what-cursor-position) ;; shows point is _before_ the B
  )

;; * An after-string and invisible property on a range of characters
;; causes cursor problems.  If point is just before all of the
;; characters with the after-string and non-nil invisible property,
;; the cursor is displayed _after_ the string which is the value of
;; the after-string property.

(progn
  (kill-buffer (get-buffer-create "test"))
  (set-buffer (get-buffer-create "test"))
  (insert "AB")
  (let ((o (make-overlay 2 3))) ;; covering the B
    (overlay-put o 'after-string "Y")
    (overlay-put o 'invisible t))
  (goto-char 2) ;; between A and B
  (display-buffer (current-buffer))
  (what-cursor-position) ;; shows point is _before_ the B
  )

;; * previous-line fails if there is no legal spot on the previous
;; line to move to.  However, backward-char works fine.  Furthermore,
;; next-line has no trouble moving down over illegal spots, so the
;; behavior is inconsistent.

(progn
  (kill-buffer (get-buffer-create "test"))
  ;; The following subexpression is necessary to exhibit the bug
  ;; reliably.  Otherwise, the test case listed just above tends to
  ;; prevent the bug from appearing.
  (progn
    (delete-other-windows)
    (sit-for 0))
  (set-buffer (get-buffer-create "test"))
  (insert "A\nB\nC\nD\nE")
  (let ((o (make-overlay 3 8))) ;; covering the "B\nC\nD"
    (overlay-put o 'intangible t))
  (goto-char 8)
  (previous-line 1)
  (display-buffer (current-buffer))
  (what-cursor-position) ;; shows point has not moved from 8
  )

;; * Column numbers as used by previous-line and next-line have
;; surprising (to the naive user) effects when overlay or text
;; properties are used which cause additional characters to be
;; displayed or prevent characters in the buffer from being displayed.
;; Strictly speaking, this may not be a bug, but it is highly
;; undesirable behavior.  There should be a way to have motion to a
;; particular column based on what is displayed rather than on what is
;; in the buffer.