Start Emacs with `emacs -Q' and evaluate the following code:

;;;;; CODE BEGIN ;;;;;

(defun test-child-frame ()
  (interactive)
  (let ((f (make-frame `((parent-frame . ,(selected-frame))
                         (internal-border-width . 3)
                         (vertical-scroll-bars . nil)
                         (menu-bar-lines . 0)
                         (tool-bar-lines . t)
                         (left . 20)
                         (top . 20)
                         (width . 50)
                         (height . 20))))
        (b (generate-new-buffer "*child-frame-test*")))
    ;; Setup the frame and the buffer.
    (with-selected-frame f
      (switch-to-buffer b)
      (with-current-buffer b
        (set-background-color "black")
        (setq mode-line-format nil)))
    (sit-for 0.3)
    ;; inhibit redisplay
    (let ((inhibit-redisplay t))
      ;; resize
      (set-frame-size f 50 10)
      ;; move
      (set-frame-position f 20 60))
    (sit-for 0.3)
    ;; clean up
    (delete-frame f)
    (kill-buffer b)))

(test-child-frame) ;; Call the function defined above.

;;;;; CODE END ;;;;;

I recorded what I saw and sent it as the attachment "bug_slow_motion.mp4".
Briefly, the child frame appeared, then moved downwards, and then resized.
The moving and resizing are very quick so I slowed down the video (about
10x slower).

There are two problems:

1. I wrap the set size and set position code in a `(let ((inhibit-redisplay
t)) ...)'.
The docstring of `inhibit-redisplay' says "Non-nil means don’t actually do
any redisplay.",
but the process of setting size and setting position is displayed.

2. In the code I gave, `set-frame-size' should be executed before
`set-frame-position'.
But in fact, the child frame was first moved, and then resized. It is in
the wrong order.


In GNU Emacs 29.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.38,
cairo version 1.17.8)
Windowing system distributor 'The X.Org Foundation', version 11.0.12101008
System Description: Arch Linux

Configured using:
 'configure --with-x-toolkit=gtk3 --with-native-compilation=aot
 --sysconfdir=/etc --prefix=/usr --libexecdir=/usr/lib
 --with-tree-sitter --localstatedir=/var --with-cairo
 --disable-build-details --with-harfbuzz --with-libsystemd
 --with-modules 'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fno-plt
 -fexceptions -Wp,-D_FORTIFY_SOURCE=2 -Wformat -Werror=format-security
 -fstack-clash-protection -fcf-protection -g
 -ffile-prefix-map=/build/emacs/src=/usr/src/debug/emacs -flto=auto'
 'LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now -flto=auto'
 'CXXFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fno-plt -fexceptions
 -Wp,-D_FORTIFY_SOURCE=2 -Wformat -Werror=format-security
 -fstack-clash-protection -fcf-protection -Wp,-D_GLIBCXX_ASSERTIONS -g
 -ffile-prefix-map=/build/emacs/src=/usr/src/debug/emacs -flto=auto''

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSYSTEMD LIBXML2 M17N_FLT MODULES NATIVE_COMP NOTIFY
INOTIFY PDUMPER PNG RSVG SECCOMP SOUND SQLITE3 THREADS TIFF
TOOLKIT_SCROLL_BARS TREE_SITTER WEBP X11 XDBE XIM XINPUT2 XPM GTK3 ZLIB

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

Major mode: ELisp/l

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-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
  blink-cursor-mode: t
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message mailcap yank-media puny dired
dired-loaddefs rfc822 mml mml-sec password-cache epa derived epg rfc6068
epg-config gnus-util text-property-search time-date mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader cl-loaddefs
comp comp-cstr warnings icons subr-x rx cl-seq cl-macs gv cl-extra
help-mode bytecomp byte-compile cl-lib sendmail rfc2047 rfc2045
ietf-drums mm-util mail-prsvr mail-utils 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 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
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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 lcms2
dynamic-setting system-font-setting font-render-setting cairo
move-toolbar gtk x-toolkit xinput2 x multi-tty make-network-process
native-compile emacs)

Memory information:
((conses 16 79379 8930)
 (symbols 48 7147 0)
 (strings 32 19698 1723)
 (string-bytes 1 576171)
 (vectors 16 16564)
 (vector-slots 8 334975 11306)
 (floats 8 33 71)
 (intervals 56 420 0)
 (buffers 984 14))