bug#53799: 27.1; Entered text sometimes appears on the preceding line

[Markus Triska <triska@metalevel.at>]

To reproduce this issue, please start Emacs with "$ emacs -Q",
and do:

    M-x report-emacs-bug RET RET C-n

In the resulting buffer, point is now placed so that the cursor is shown
on the letter "I" in the following text:

   "If Emacs crashed, and you have the Emacs process ..."

If you then type any letter with the intention of entering text on this
line, it unexpectedly appears in the preceding line. For example, with
point at the "I" of "If", when I enter "a", I get:

    a
    If

whereas one would expect to obtain:

    aIf

This behaviour seems to only occur on this specific line, and therefore
seems unusual.

This is only a very minor issue, yet still a bit surprising, especially
since "I" remains highlighted the entire time, also if multiple
characters are entered, all appearing on the preceding line, such as in:

    abcd
    If

Thank you a lot!
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