Recipe to reproduce:

1. Set `diff-font-lock-prettify' to `t' and enable `outline-minor-mode'
in `diff-mode'; For example:

(add-hook 'diff-mode-hook
	  (lambda ()
            (setq diff-font-lock-prettify t)
	    (outline-minor-mode)))

2. In a *vc-change-log* buffer for emacs sources, find the commit with
hash a8de88e330. Press = (`log-view').

3. Press C-c @ C-t (`outline-hide-body') while in the *vc-diff*
buffer. Observe that outline headers are splitted in multiple lines.


In GNU Emacs 28.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.24, cairo version 1.16.0)
 of 2021-09-13 built on carbon
Repository revision: 8c80430824d52e5dfae97db2669b419621817956
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Debian GNU/Linux 11 (bullseye)

Configured using:
 'configure --with-native-compilation'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBXML2 M17N_FLT MODULES NATIVE_COMP NOTIFY
INOTIFY PDUMPER PNG RSVG SECCOMP SOUND THREADS TIFF TOOLKIT_SCROLL_BARS
X11 XDBE XIM XPM GTK3 ZLIB

Important settings:
  value of $LANG: fr_FR.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Message

Minor modes in effect:
  goto-address-mode: t
  highlight-changes-visible-mode: t
  show-paren-mode: t
  shell-dirtrack-mode: t
  minions-mode: t
  mml-mode: t
  desktop-save-mode: t
  save-place-mode: t
  electric-pair-mode: t
  icomplete-mode: t
  global-so-long-mode: t
  global-auto-revert-mode: t
  auto-insert-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-layout-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tab-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  window-divider-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  auto-fill-function: message-do-auto-fill
  transient-mark-mode: t
  abbrev-mode: t

Load-path shadows:
/home/matthias/.config/emacs/elpa/transient-20210919.1006/transient hides /usr/local/share/emacs/28.0.50/lisp/transient
/home/matthias/.config/emacs/elpa/flymake-1.2.1/flymake hides /usr/local/share/emacs/28.0.50/lisp/progmodes/flymake
/home/matthias/.config/emacs/elpa/project-0.7.1/project hides /usr/local/share/emacs/28.0.50/lisp/progmodes/project
/home/matthias/.config/emacs/elpa/dictionary-20201001.1727/dictionary hides /usr/local/share/emacs/28.0.50/lisp/net/dictionary

Features:
(shadow emacsbug sendmail nndoc gnus-dup url-cache crm debbugs-gnu
debbugs soap-client url-http url-auth url-gw rng-xsd xsd-regexp
debbugs-browse dabbrev cl-print cus-start texinfo texinfo-loaddefs
help-fns radix-tree js add-log log-view pcvs-util misearch multi-isearch
smerge-mode diff gnus-fun flow-fill mm-archive view sort smiley
gnus-cite mail-extr gnus-async gnus-bcklg qp gnus-ml disp-table
gnus-topic nndraft nnmh nnfolder utf-7 epa-file gnutls network-stream
nsm gnus-agent gnus-srvr gnus-score score-mode nnvirtual gnus-msg
gnus-cache hl-line conf-mode whitespace ox-odt rng-loc rng-uri rng-parse
rng-match rng-dt rng-util rng-pttrn nxml-parse nxml-ns nxml-enc xmltok
nxml-util ox-latex ox-icalendar ox-html table ox-ascii ox-publish ox
org-element avl-tree generator ol-eww eww xdg url-queue mm-url ol-rmail
ol-mhe ol-irc ol-info ol-gnus nnselect gnus-search eieio-opt speedbar
ezimage dframe gnus-art mm-uu mml2015 mm-view mml-smime smime dig
gnus-sum shr kinsoku svg dom ol-docview doc-view image-mode exif
ol-bibtex bibtex ol-bbdb ol-w3m flyspell goto-addr bug-reference
dired-aux vc-dir follow mule-util display-line-numbers hilit-chg paren
vc-mtn vc-hg vc-bzr vc-src vc-sccs vc-svn vc-cvs vc-rcs vc jka-compr
bash-completion shell eglot array jsonrpc ert ewoc debug backtrace xref
flymake-proc flymake compile pcase project imenu avoid minions
carbon-custom cus-edit cus-load gnus-demon nntp gnus-group gnus-undo
gnus-start gnus-dbus dbus xml gnus-cloud nnimap nnmail mail-source utf7
netrc parse-time iso8601 gnus-spec gnus-win nnoo gnus-int gnus-range
message rmc puny rfc822 mml mml-sec epa derived epg rfc6068 epg-config
mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils
mailheader gnus nnheader gnus-util rmail rmail-loaddefs rfc2047 rfc2045
ietf-drums mail-utils mm-util mail-prsvr wid-edit gnus-dired dired-x
dired dired-loaddefs org-capture org-refile org ob ob-tangle ob-ref
ob-lob ob-table ob-exp org-macro org-footnote org-src ob-comint
org-pcomplete pcomplete comint ansi-color ring org-list org-faces
org-entities time-date org-version ob-emacs-lisp ob-core ob-eval
org-table ol org-keys org-compat org-macs org-loaddefs format-spec
find-func cal-menu calendar cal-loaddefs dictionary link connection
advice markdown-mode edit-indirect color thingatpt noutline outline
skeleton find-file vc-git diff-mode easy-mmode vc-dispatcher ispell
desktop frameset server bookmark text-property-search pp saveplace
elec-pair edmacro kmacro icomplete so-long autorevert filenotify
autoinsert cc-mode cc-fonts cc-guess cc-menus cc-cmds cc-styles cc-align
cc-engine cc-vars cc-defs generic-x face-remap proof-site
proof-autoloads comp comp-cstr warnings rx cl-extra help-mode info
package browse-url url url-proxy url-privacy url-expand url-methods
url-history url-cookie url-domsuf url-util mailcap url-handlers
url-parse auth-source cl-seq eieio eieio-core cl-macs eieio-loaddefs
password-cache json subr-x map url-vars seq byte-opt gv bytecomp
byte-compile cconv cl-loaddefs cl-lib iso-transl tooltip eldoc electric
uniquify ediff-hook vc-hooks lisp-float-type mwheel term/x-win x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch easymenu
timer select scroll-bar mouse jit-lock font-lock syntax font-core
term/tty-colors frame minibuffer 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 cl-preloaded nadvice button
loaddefs faces cus-face macroexp files window text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote threads dbusbind inotify lcms2
dynamic-setting system-font-setting font-render-setting cairo
move-toolbar gtk x-toolkit x multi-tty make-network-process
native-compile emacs)

Memory information:
((conses 16 1700523 255710)
 (symbols 48 38830 103)
 (strings 32 227548 67913)
 (string-bytes 1 7153152)
 (vectors 16 75900)
 (vector-slots 8 1725779 101458)
 (floats 8 2990 1100)
 (intervals 56 136558 2364)
 (buffers 992 150))

-- 
Matthias