bug#30519: 26.0.91; Emacs hangs with flyspell overlay

[Nick Helm]

Emacs hangs when point enters a word that is highlighted by flyspell. 

I cannot reproduce this on an NS build, so I originally thought it was
limited to the mac-port. However the maintainer reproduced it with the
emacs-26 branch, both on GTK+ and Athena toolkits. 

Recipe:

Emacs -Q
(setq mouse-highlight 1)
(flyspell-mode 1)
"Misspetl wd " ;enter some text that becomes marked by flyspell
;place the mouse pointer over the word "Misspetl"
;hold the left cursor key until point moves back to the word "Misspetl"

Emacs hangs with repeating error "Marker does not point anywhere". You
can clear the hang by moving the mouse pointer. 



In GNU Emacs 26.0.91 (build 1, x86_64-apple-darwin17.4.0, NS appkit-1561.20 
Version 10.13.3 (Build 17D47))
 of 2018-02-19 built on oberon.local
Windowing system distributor 'Apple', version 10.3.1561
Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Emacs start-up time: 0.2 seconds
Package html2text is obsolete!

Configured using:
 'configure --with-ns --with-gnutls=no'

Configured features:
JPEG NOTIFY ACL LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS LCMS2

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

Major mode: Text

Minor modes in effect:
  savehist-mode: t
  global-eldoc-mode: t
  mouse-wheel-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
  visual-line-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow face-remap emacsbug mu4e desktop frameset mu4e-speedbar speedbar
sb-image ezimage dframe mu4e-main mu4e-view cal-menu calendar
cal-loaddefs thingatpt browse-url comint ansi-color ring mu4e-headers
mu4e-compose mu4e-context mu4e-draft mu4e-actions ido rfc2368 smtpmail
auth-source cl-seq eieio eieio-core cl-macs eieio-loaddefs sendmail
mu4e-mark mu4e-message flow-fill html2text mu4e-proc mu4e-utils doc-view
jka-compr image-mode mu4e-lists mu4e-vars message rmc puny seq byte-opt
bytecomp byte-compile cconv format-spec rfc822 mml mml-sec
password-cache epa derived epg epg-config gnus-util rmail rmail-loaddefs
mm-decode mm-bodies mm-encode mail-parse rfc2231 rfc2047 rfc2045 mm-util
ietf-drums mail-prsvr mailabbrev mail-utils gmm-utils mailheader hl-line
cl mu4e-meta time dired-x easymenu dired dired-loaddefs pcase savehist
nh-macdict easy-mmode iso-transl edmacro kmacro cl-loaddefs cl-lib gv
plain-theme time-date tooltip eldoc electric uniquify ediff-hook
vc-hooks lisp-float-type mwheel term/ns-win ns-win ucs-normalize
mule-util term/common-win tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page menu-bar rfn-eshadow isearch timer select
scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame cl-generic cham georgian utf-8-lang misc-lang
vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms cp51932
hebrew greek romanian slovak czech european ethiopic indian cyrillic
chinese composite charscript charprop case-table epa-hook jka-cmpr-hook
help simple abbrev obarray minibuffer cl-preloaded nadvice loaddefs
button faces cus-face macroexp files text-properties overlay sha1 md5
base64 format env code-pages mule custom widget hashtable-print-readable
backquote kqueue cocoa ns lcms2 multi-tty make-network-process emacs)

Memory information:
((conses 16 249898 10278)
 (symbols 48 24633 1)
 (miscs 40 65 278)
 (strings 32 43132 1725)
 (string-bytes 1 1188676)
 (vectors 16 39850)
 (vector-slots 8 764479 9432)
 (floats 8 87 133)
 (intervals 56 237 0)
 (buffers 992 11))