bug#40849: 26.3; count-screen-lines returns incorrect result when truncated lines are counted

[Alexander Miller]

Starting from emacs -q run this:

(progn   (select-window
   (display-buffer-in-side-window
    (get-buffer-create (generate-new-buffer-name "*Test*"))
    '((side . left))))
  (shrink-window-horizontally  (- (window-width) 10))
  (insert (make-string 10 ?A) "\n"
          (make-string 11 ?A) "\n"
          (make-string 10 ?A))
  (goto-char 0)
  (forward-line 1)
  (count-screen-lines (window-start) (line-end-position)))

The result will be 3 even though point is on the second line. If the
window is instead made large enough to accommodate the entire length of
all inserted lines the result is 2.

The expected behavior is to return 2 in both situations.

In GNU Emacs 26.3 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.10)
 of 2019-08-29 built on juergen
Windowing system distributor 'The X.Org Foundation, version 11.0.12008000
Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Mark set
3 (#o3, #x3, ?\C-c)
Making completion list... [2 times]

Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --with-x-toolkit=gtk3 --with-xft --with-modules
 'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fno-plt'
 CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS GLIB
NOTIFY ACL GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS LIBSYSTEMD LCMS2

Important settings:
  value of $LC_COLLATE: en_GB.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
  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
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny seq byte-opt gv
bytecomp byte-compile cconv cl-loaddefs cl-lib dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail rmail-loaddefs mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils elec-pair time-date
mule-util tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type 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 elisp-mode lisp-mode prog-mode register page menu-bar
rfn-eshadow isearch timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core term/tty-colors frame cl-generic 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 charscript charprop
case-table epa-hook jka-cmpr-hook help simple abbrev obarray minibuffer
cl-preloaded nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote threads dbusbind
inotify lcms2 dynamic-setting system-font-setting font-render-setting
move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 95003 5589)
 (symbols 48 20384 1)
 (miscs 40 64 118)
 (strings 32 28426 1024)
 (string-bytes 1 746496)
 (vectors 16 13938)
 (vector-slots 8 502078 10192)
 (floats 8 56 61)
 (intervals 56 240 7)
 (buffers 992 13))