bug#24567: 25.1; Cursor flickering with large margin

[Vasilij Schneidermann <v.schneidermann@gmail.com>]

Hello,

I've noticed this bug in an IRC buffer with a minor mode enabled that
sets up the window margins to make the contents appear as if they have
been soft-wrapped with the value of `fill-column'.  Quick typing makes
the cursor disappear and reappear erratically.  Restoring the window
margins to their original value of 0 makes it go away again.

To reproduce this in a minimal Emacs session, I've used the following in
the scratch buffer:

    (blink-cursor-mode -1)
    (set-window-margins nil 0 (- (window-width) 50))

Then one can hold down a key and watch the cursor carefully as it starts
flickering until you let go of it.

Is anyone else able to reproduce this?  What would be suitable debugging
steps to find out the reason behind this glitch?  Could it be the fault
of my graphics setup?



In GNU Emacs 25.1.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.20.9)
 of 2016-09-18 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.11804000
Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --with-x-toolkit=gtk3 --with-xft
 'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe
 -fstack-protector-strong' CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF GSETTINGS
NOTIFY ACL GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11

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

Major mode: Lisp Interaction

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
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent messages:
Making completion list... [2 times]
nil
t
Making completion list...
Quit
Making completion list... [2 times]
Type C-x 1 to delete the help window.
230 (#o346, #xe6, ?æ)
t
Undo! [3 times]
t

Load-path shadows:
None found.

Features:
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mule-util tooltip eldoc electric uniquify ediff-hook vc-hooks
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image regexp-opt fringe tabulated-list newcomment elisp-mode lisp-mode
prog-mode register page menu-bar rfn-eshadow timer select scroll-bar
mouse jit-lock font-lock syntax facemenu font-core frame cl-generic cham
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hashtable-print-readable backquote dbusbind inotify dynamic-setting
system-font-setting font-render-setting move-toolbar gtk x-toolkit x
multi-tty make-network-process emacs)

Memory information:
((conses 16 94201 6780)
 (symbols 48 20135 0)
 (miscs 40 51 196)
 (strings 32 16654 4240)
 (string-bytes 1 458316)
 (vectors 16 12355)
 (vector-slots 8 438292 5950)
 (floats 8 169 502)
 (intervals 56 885 68)
 (buffers 976 20))