bug#71591: 30.0.50; PGTK: Terrible input lag with display scaling

[Jarno Malmari]

With a 4k display and 150 % display scaling there is a noticeable input
lag when moving the point around in fullscreen. Very sluggish
feel. Shrinking the window helps.

Besides "sluggish feel" I couldn't figure out easily measurable metric
for this. One way is to clock how long it takes to use your keyboard
repeat with `next-line' to reach the end of a couple pages long buffer.

This was tested with PGTK build on KDE Plasma 6 running Wayland. X build
running in XWayland does not have this problem even if configured to be
"Scaled by the system" from the system display configuration UI.

The scale option that affects this can also be changed with these commands:

    kscreen-doctor output.${OUTPUT_NAME}.scale.1.5
    kscreen-doctor output.${OUTPUT_NAME}.scale.1.0

Applies also to emacs 29.

-- 

In GNU Emacs 30.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 3.24.42, cairo version 1.18.0) of 2024-06-16 built on ***
Repository revision: a39f6480ea5b6d3f81db90808123cdeb54790787
Repository branch: master
System Description: Arch Linux

Configured using:
 'configure --with-pgtk'

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

Important settings:
  value of $LC_MONETARY: fi_FI.UTF-8
  value of $LC_NUMERIC: fi_FI.UTF-8
  value of $LC_TIME: fi_FI.UTF-8
  value of $LANG: en_GB.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

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
  minibuffer-regexp-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 compile comint ansi-osc ansi-color ring comp-run
bytecomp byte-compile comp-common rx 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 subr-x
mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils
mailheader cl-loaddefs 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/pgtk-win pgtk-win term/common-win touch-screen pgtk-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
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 gtk pgtk
lcms2 multi-tty move-toolbar make-network-process native-compile emacs)

Memory information:
((conses 16 61800 9334) (symbols 48 6696 0) (strings 32 17531 4045)
 (string-bytes 1 505683) (vectors 16 11602)
 (vector-slots 8 140314 7100) (floats 8 22 2) (intervals 56 247 0)
 (buffers 992 11))