bug#50337: 28.0.50; (while-no-input (redisplay) forms...) block behaves differently when window-system is pgtk

[Daniel Semyonov via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

This was tested on both a build configured with "--without-x
--with-pgtk" and a build configured with "--with-x --with-x-toolkit=no"
(my previous configuration before switching to the pgtk branch). Both
were built from the pgtk branch.

In the following steps I load a package I'm developing called
Vcomplete[1] which uses the following function to update the
'*Completions*' buffer when providing input to the minibuffer (the
'*Completions*' buffer also opens automatically when starting minibuffer
completion):

(defun vcomplete--update-in-minibuffer (&rest _args)
  "Update the completion list when completing in a minibuffer."
  (while-no-input
    (redisplay)
    (unless (memq this-command vcomplete-no-update-commands)
      (minibuffer-completion-help))))

For more details look at the package, specifically the section called
'Visual completion mode' (it's not very complex and I don't want to make
this report too long, I can provide more details if necessary).
Also, the effect is much more apparent when 'completions-detailed' is
set to t (presumably because it takes longer to display the completions).

The following steps are performed on both builds:

1. emacs -Q
2. M-x load-file RET ~/src/vcomplete/vcomplete.el RET
3. M-x vcomplete-mode RET
4. C-h f bac (combination of letters which correspond to completions)

On the X build, as soon as I start typing letters in step 4 the
'*Completions*' buffer immediately updates; on the pgtk build there is a
noticable (maybe half a second) delay during which I also can't see the
letters I'm typing into the minibuffer, in addition to the first few
letters being entered into the minibuffer twice (the amount of letters
duplicated changes).

The behavior on the X build lines up with the documentation strings of
'redisplay' and 'while-no-input', so I'm assuming this is a bug in pgtk
redisplay code (although my only knowledge of redisplay comes from
reading documentation, so I might be off).

Hope I did everything right, this is the first bug report I'm
submitting, thanks!

Daniel

[1] https://git.sr.ht/~dsemy/vcomplete


In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.29, cairo version 1.16.0)
 of 2021-08-31 built on coldharbour
Repository revision: aa437f6a59337d980795370d55edebb029ecf10b
Repository branch: master
Windowing system distributor 'System Description: void

Configured using:
 'configure --without-dbus --without-gsettings --without-x --with-pgtk
 --prefix=/opt/Emacs'

Configured features:
CAIRO FREETYPE GIF GLIB GMP GNUTLS HARFBUZZ JPEG JSON LCMS2 LIBSYSTEMD
LIBXML2 MODULES NOTIFY INOTIFY PDUMPER PGTK PNG RSVG SECCOMP SOUND
THREADS TIFF TOOLKIT_SCROLL_BARS XIM GTK3 ZLIB

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

Major mode: Help

Minor modes in effect:
  global-git-commit-mode: t
  magit-auto-revert-mode: t
  shell-dirtrack-mode: t
  emms-mode-line-mode: t
  emms-playing-time-display-mode: t
  emms-playing-time-mode: t
  pdf-occur-global-minor-mode: t
  show-paren-mode: t
  electric-pair-mode: t
  repeat-mode: t
  swsw-mode: t
  global-auto-revert-mode: t
  delete-selection-mode: t
  vcomplete-mode: t
  display-time-mode: t
  minibuffer-line-mode: t
  minibuffer-electric-default-mode: t
  tooltip-mode: t
  global-eldoc-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
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
/home/daniel/etc/emacs/elpa/transient-0.3.6/transient hides /opt/Emacs/share/emacs/28.0.50/lisp/transient

Features:
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git-commit rx log-edit pcvs-util add-log magit-core magit-autorevert
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mule custom widget hashtable-print-readable backquote threads inotify
dynamic-setting font-render-setting cairo move-toolbar gtk x-toolkit
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Memory information:
((conses 16 634923 559779)
 (symbols 48 37441 3)
 (strings 32 180878 95332)
 (string-bytes 1 7865170)
 (vectors 16 76290)
 (vector-slots 8 1556469 156147)
 (floats 8 732 742)
 (intervals 56 8164 3393)
 (buffers 992 35))