bug#73234: 30.0.91; completion-preview-mode doesn't trigger for case-insensitive capf

[Morgan Willcock <morgan@ice9.digital>]

I believe there is an issue with completion-preview-mode in that it
doesn't seem to find all completions for a case-insensitive completion
at point function.

Evaluating the code below in `emacs -Q` will put the current buffer into
fundamental mode with a case-insensitive completion at point function:

  (defun test-completion-at-point-function ()
    "Complete the symbol at point."
    (pcase (bounds-of-thing-at-point 'symbol)
      (`(,beg . ,end)
       (list beg end
             (completion-table-case-fold
              (completion-table-dynamic
               (lambda (string)
                 (list "TestSymbol"))))))))

  (fundamental-mode)
  (remove-hook 'completion-at-point-functions #'tags-completion-at-point-function)
  (add-hook 'completion-at-point-functions #'test-completion-at-point-function)
  (completion-preview-mode 1)

Once the code is evaluated:

- Typing "tes" into the buffer will not display the completion preview.

- The default completion mechanism (as initiated with complete-symbol
  via C-M-i) will work with no problems, and replace "tes" with the
  completion "TestSymbol".

- Typing "Tes" into the buffer will display the completion preview.

I would expect that the completion preview should be displayed for both
"tes" and "Tes" since a completion is available to use in both cases.



In GNU Emacs 30.0.91 (build 2, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.16.0, Xaw3d scroll bars) of 2024-09-12 built on inspiron
Windowing system distributor 'The X.Org Foundation', version 11.0.12101007
System Description: Debian GNU/Linux 12 (bookworm)

Configured using:
 'configure --with-native-compilation=aot --with-xml2
 --with-x-toolkit=lucid'

Configured features:
CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG
LIBSELINUX LIBXML2 MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG
SECCOMP SOUND SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS WEBP X11 XAW3D
XDBE XIM XINPUT2 XPM LUCID ZLIB

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

Major mode: ELisp/d

Minor modes in effect:
  server-mode: t
  global-corfu-mode: t
  corfu-mode: t
  jabber-activity-mode: t
  etags-regen-mode: t
  which-key-mode: t
  global-devil-mode: t
  devil-mode: t
  erc-ring-mode: t
  erc-netsplit-mode: t
  erc-menu-mode: t
  erc-list-mode: t
  erc-irccontrols-mode: t
  erc-move-to-prompt-mode: t
  erc-readonly-mode: t
  erc-imenu-mode: t
  erc-button-mode: t
  erc-fill-mode: t
  erc-stamp-mode: t
  erc-spelling-mode: t
  erc-track-mode: t
  erc-match-mode: t
  erc-autojoin-mode: t
  erc-networks-mode: t
  savehist-mode: t
  minibuffer-electric-default-mode: t
  minibuffer-depth-indicate-mode: t
  ido-everywhere: t
  recentf-mode: t
  global-display-fill-column-indicator-mode: t
  display-fill-column-indicator-mode: t
  global-hl-line-mode: t
  display-time-mode: t
  flyspell-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  minibuffer-regexp-mode: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
/home/mwillcock/.emacs.d/custom/powershell.el/powershell hides /home/mwillcock/.emacs.d/custom/powershell
/home/mwillcock/.emacs.d/elpa/which-key-3.6.1/which-key hides /usr/local/share/emacs/30.0.91/lisp/which-key
/home/mwillcock/.emacs.d/elpa/modus-themes-4.5.0/theme-loaddefs hides /usr/local/share/emacs/30.0.91/lisp/theme-loaddefs

Features:
(shadow sort ecomplete mail-extr emacsbug cursor-sensor server cal-iso
autorevert face-remap init corfu sly sly-completion sly-buttons
sly-messages sly-common apropos etags fileloop arc-mode archive-mode
hyperspec powershell php-mode mode-local consult-imenu consult php-face
php php-project jabber jabber-ourversion hippie-exp hexrgb fsm sgml-mode
facemenu srv dns starttls tls goto-addr yaml-mode markdown-mode lua-mode
advice edmacro kmacro etags-regen kixtart-docstrings cape kixtart-mode
tempo org-msg let-alist color 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
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htmlize gnus-msg gnus-icalendar icalendar gnus-dired gnus-cite which-key
devil delight comp comp-cstr ags-mode speedbar ezimage dframe shadowfile
eglot jsonrpc xref flymake diff ert ewoc debug backtrace warnings python
project cc-mode cc-fonts cc-guess cc-menus cc-cmds cc-styles cc-align
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erc-sasl-scram-sha-256 erc-sasl-scram-sha-1 erc-sasl-external
erc-sasl-plain sasl-scram-sha256 sasl-scram-rfc sasl-scram-sha-1 rfc2104
hex-util sasl sasl-anonymous sasl-login sasl-plain erc-spelling
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org-src sh-script smie treesit executable ob-comint org-pcomplete
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ob-emacs-lisp ob-core ob-eval org-version org-cycle org-table ol
org-fold org-fold-core org-keys oc org-loaddefs thingatpt org-compat
org-macs autoinsert compile bookmark savehist crm minibuf-eldef mb-depth
ido tramp-cache time-stamp tramp-sh tramp rx trampver tramp-integration
files-x tramp-message tramp-compat xdg format-spec tramp-loaddefs
recentf tree-widget shell pcomplete comint ansi-osc ansi-color ring
easy-mmode display-fill-column-indicator hl-line time gnus-group
gnus-undo gnus-start gnus-dbus dbus gnus-cloud nnimap nnmail mail-source
utf7 nnoo parse-time iso8601 gnus-spec gnus-int gnus-range gnus-win gnus
nnheader range cus-edit pp cus-load wid-edit dictionary
external-completion dictionary-connection flyspell ispell shr pixel-fill
kinsoku url-file svg xml dom modus-vivendi-theme modus-themes mm-view
mml-smime smime gnutls dig smtpmail message sendmail yank-media puny
dired dired-loaddefs rfc822 mml mml-sec epa derived epg rfc6068
gnus-util text-property-search time-date mm-decode mm-bodies mm-encode
mail-parse rfc2231 rfc2047 rfc2045 mm-util ietf-drums mail-prsvr
mailabbrev mail-utils gmm-utils mailheader epg-config help-mode dabbrev
find-func cal-menu calendar cal-loaddefs desktop frameset pcase
cape-autoloads consult-autoloads corfu-autoloads delight-autoloads
do-at-point-autoloads fsm-autoloads htmlize-autoloads lua-mode-autoloads
markdown-mode-autoloads modus-themes-autoloads package-lint-autoloads
php-mode-autoloads rainbow-mode-autoloads renpy-mode-autoloads
sly-autoloads totp-auth-autoloads base32-autoloads info
vertico-autoloads wgrep-autoloads which-key-autoloads
yaml-mode-autoloads package browse-url url url-proxy url-privacy
url-expand url-methods url-history url-cookie generate-lisp-file
url-domsuf url-util mailcap url-handlers url-parse auth-source cl-seq
eieio eieio-core cl-macs icons password-cache json subr-x map byte-opt
gv bytecomp byte-compile url-vars cl-loaddefs cl-lib rmc iso-transl
tooltip cconv eldoc paren electric uniquify ediff-hook vc-hooks
lisp-float-type elisp-mode mwheel term/x-win x-win term/common-win x-dnd
touch-screen tool-bar dnd fontset image regexp-opt fringe tabulated-list
replace newcomment text-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
nadvice seq simple cl-generic indonesian philippine 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 emoji-zwj charscript charprop
case-table epa-hook jka-cmpr-hook help abbrev obarray oclosure
cl-preloaded button loaddefs theme-loaddefs faces cus-face macroexp
files window text-properties overlay sha1 md5 base64 format env
code-pages mule custom widget keymap hashtable-print-readable backquote
threads dbusbind inotify dynamic-setting system-font-setting
font-render-setting cairo x-toolkit xinput2 x multi-tty move-toolbar
make-network-process native-compile emacs)

Memory information:
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 (strings 32 260961 5577) (string-bytes 1 7128489) (vectors 16 148988)
 (vector-slots 8 2543291 190764) (floats 8 781 818)
 (intervals 56 123265 150) (buffers 984 21))