bug#33858: 26.1; Scrolling up fails

[Andrew Kurn]

--text follows this line--

Dear Bug-Hunters,

Here's a holiday treat for you:  a weird bug.

Here's the bug:  When I scroll up (using my own function
a-unscroll (q.v.)) and when point is about to scroll down
off the bottom of the screen, the scroll action fails, and
the line with point is repositioned to scroll-step lines
above the bottom of the screen.

This bug does not appear with emacs -Q.

I have whittled down my init file to see which statement
triggers the bug.  Here is the shortest one for which the
but appears:

--

(global-set-key [(f12)] 'a-scroll)
(global-set-key [(shift f12)] 'a-unscroll)
(defun a-scroll (p) "Scroll this window up (fd) one line."
  (interactive "p")
  (let (( scroll-preserve-screen-position nil))
    (scroll-up p)))
(defun a-unscroll (p) "Scroll this window down (back) one line."
  (interactive "p")
  (let (( scroll-preserve-screen-position nil))
    (scroll-up (- p))))

(setq scroll-step 3)
(setq next-line-add-newlines nil)
(setq make-backup-files nil)
(setq auto-save-timeout 300)
(setq auto-save-interval 3000)
(setq track-eol t)
(setq line-move-visual nil)
(setq scroll-preserve-screen-position t)
(setq c-tab-always-indent nil)
;;(setq printer-name "//earth/hp pcl 6")
;;(setq ps-printer-name printer-name)

(setq-default case-fold-search nil)
(setq scroll-bar-adjust-thumb-portion nil)
(add-to-list 'default-frame-alist
             '(font . "Nimbus Mono-13:bold"))

--

If I remove the add-to-list, the bug goes away.

Weird, eh?

Also, if I say emacs -Q and then eval-buffer, no bug.

Happy hunting,
Andrew






In GNU Emacs 26.1 (build 1, x86_64-apple-darwin14.5.0, NS appkit-1348.17 
Version 10.10.5 (Build 14F2511))
 of 2018-05-30 built on builder10-10.porkrind.org
Windowing system distributor 'Apple', version 10.3.1504
Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Mark set [18 times]
Type C-x 1 to remove help window.  
<f8> is undefined
Type "q" in help window to restore its previous buffer.

Configured using:
 'configure --with-ns '--enable-locallisppath=/Library/Application
 Support/Emacs/${version}/site-lisp:/Library/Application
 Support/Emacs/site-lisp' --with-modules'

Configured features:
NOTIFY ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS MODULES THREADS

Important settings:
  value of $LANG: en_CA.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Apropos

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-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
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

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Memory information:
((conses 16 209489 10151)
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