bug#53798: 27.1; OSX: (redisplay) does not reliably redisplay

[Markus Triska <triska@metalevel.at>]

To reproduce this issue, please start Emacs with:

    $ emacs -Q

and evaluate the following form:

    (while t
      (insert "\n" (make-string 50 ?a))
      (redisplay)
      (sit-for 0.1))

At first, this works completely as intended: We see a growing number of
lines in the buffer, and we are shown each fresh line as it appears:

    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    ...

However, as soon as any key is pressed, the output becomes erratic in
the sense that for long stretches of time, we see no new lines at all,
and then several of them appear immediately at the same time.

The snippet uses (redisplay) after writing each line in order to show
the line as it appears. I therefore expect to continue to see, also when
a key is pressed, each line immediately after it is written in the
buffer, not batches of multiple lines to be shown after several of them
have already been written. Is there any way to obtain this behaviour?

Thank you a lot!

All the best,
Markus

In GNU Emacs 27.1 (build 1, x86_64-apple-darwin15.3.0, X toolkit, Xaw scroll bars)
 of 2020-12-12 built on mt-macbook
Windowing system distributor 'The X.Org Foundation', version 11.0.11502000
System Description:  Mac OS X 10.11.3

Configured using:
 'configure --prefix=/opt/local --disable-silent-rules --without-ns
 --without-dbus --without-gconf --without-libotf --without-m17n-flt
 --with-gmp --with-gnutls --with-json --with-xml2 --with-modules
 --infodir /opt/local/share/info/emacs --with-x-toolkit=lucid
 --without-xaw3d --without-imagemagick --with-xpm --with-jpeg
 --with-tiff --with-gif --with-png --with-lcms2 --without-rsvg
 --with-xft 'CFLAGS=-pipe -Os -arch x86_64'
 CPPFLAGS=-I/opt/local/include 'LDFLAGS=-L/opt/local/lib
 -Wl,-headerpad_max_install_names -lfreetype -lfontconfig -Wl,-no_pie
 -arch x86_64''

Configured features:
XPM JPEG TIFF GIF PNG GSETTINGS GLIB NOTIFY KQUEUE ACL GNUTLS LIBXML2
FREETYPE HARFBUZZ XFT ZLIB TOOLKIT_SCROLL_BARS LUCID X11 XDBE XIM
MODULES THREADS JSON PDUMPER LCMS2 GMP

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