bug#38330: 27.0.50; cursor vanishes at EOL with :extend t under certain conditions

[Ingo Lohmar]

This is a minimal working example for the bug described here:
https://lists.gnu.org/archive/html/emacs-orgmode/2019-10/msg00180.html
The main work has been done by Mr McDowell:
https://lists.gnu.org/archive/html/emacs-orgmode/2019-11/msg00106.html

Here is a MWE from 'emacs -Q', X11(lucid):

- Create a buffer "test" and "M-x org-mode", insert the following for a
code block:

* test
#+BEGIN_SRC sh
end
#+END_SRC

- In the *scratch* buffer, adjust the org-block face as follows:

(face-spec-set 'org-block '((t :background "grey80" :extend t)))

- Placing the cursor at EOL (after "end") in the code-block line, it
  should remain visible (as expected).

- In the org-mode buffer, "M-x org-indent-mode" (which, I think, works
  by setting the `line-prefix' property).  Now, the cursor in the code
  block line becomes invisible at EOL.

The relevant ingredients are the :extend property, a background face
(presumably because we need a face attribute that is actually
extendable), and org-indent-mode (I suppose for the `line-prefix'
property).

In GNU Emacs 27.0.50 (build 3, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll 
bars)
 of 2019-11-17 built on kenko
Repository revision: 3dcf06bfa035c3caaeac5ce5c585dbfc07f31331
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description: Debian GNU/Linux bullseye/sid

Recent messages:
Mark set
nil [3 times]
Mark set
nil [3 times]
GC took 0.213s
nil [7 times]
/home/il/.emacs.d/elpa/org-20191111/org-faces.el [2 times]
GC took 0.234s
Mark activated
GC took 0.235s

Configured using:
 'configure --with-imagemagick --without-gsettings --without-gconf
 --with-x-toolkit=lucid --with-modules'

Configured features:
XAW3D XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GLIB NOTIFY
INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF
XFT ZLIB TOOLKIT_SCROLL_BARS LUCID X11 XDBE XIM MODULES THREADS
LIBSYSTEMD JSON PDUMPER LCMS2 GMP

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

Major mode: Org

[... tested with emacs -Q in another instance ...]