bug#43033: 27.1; Two zero-width overlays with before- and after-string property

[Eli Zaretskii <eliz@gnu.org>]

tags 43033 notabug
thanks

> From: Ikumi Keita <ikumi@ikumi.que.jp>
> Date: Tue, 25 Aug 2020 14:26:40 +0900
> 
> (setq ov1 (make-overlay (point-max) (point-max)))
> (setq ov2 (make-overlay (point-max) (point-max)))
> (overlay-put ov1 'after-string "AFTER")
> (overlay-put ov2 'before-string "before")
> --------------------------------------------------
> 2. emacs -Q -l /tmp/foo.el
> 3. Expected result: *scrach* buffer displays "beforeAFTER" in it
>    Actual result: *scrach* buffer displays "AFTERbefore" in it

This is the expected (albeit somewhat counter-intuitive) behavior:
Emacs always sorts after-strings so that they come in front of
before-strings, except if they are from the same overlay.  This
behavior remained unchanged since Emacs 21.1.

> [How this behavior affects my usage of overlay]
> When the minibuffer has a zero-width overlay at the EOB with
> before-string property, `set-minibuffer-message' displays that string
> after, not before, the echo area message shown by that function.

This is by design: we want the minibuffer message to be as visible as
possible, so other overlays are pushed to the right (where they could
be hidden if the screen space is insufficient).