bug#58122: 28.2; overlay before/after-string does not render more than one fringe display spec

[Eli Zaretskii]

tags 58122 wontfix
thanks

> Date: Tue, 27 Sep 2022 11:05:10 -0700
> From: "Josh Caswell" <emacs@woolsweater.net>
> 
> An overlay `before-string' or `after-string' property with both left and 
> right fringe display specs only seems to render the first spec.
> 
> *Demonstration*:
> - Launch GUI Emacs with the -q flag
> - In an Elisp buffer, define these two commands
> 
> (defun insert-two-squares ()
>   (interactive)
>   (insert (propertize " " 'display [(left-fringe hollow-square)
>                                   (right-fringe hollow-square)])))
> 
> (defun overlay-two-squares (right-first)
>   (interactive "P")
>   (remove-overlays)
>   (let ((ov (make-overlay (point) (point)))
>       (specs [(left-fringe hollow-square)
>                (right-fringe hollow-square)]))
>     (overlay-put ov 'before-string
>                (propertize " "
>                            'display
>                            (if right-first
>                                (reverse specs)
>                              specs)))))
> 
> - Switch to a new test buffer
> - Invoke `insert-two-squares'; observe that both left and right fringes 
> display a hollow square
> - Delete the inserted character (fringe bitmaps disappear)
> - Invoke `overlay-two-squares'; observe that only the left fringe displays a 
> hollow square
> - Invoke `overlay-two-squares' prefixed with `C-u'; observe that only the 
> _right_ fringe displays the hollow square
> 
> The same behavior is seen if the overlay property is `after-string' rather 
> than `before-string'.
> 
> *Expectation*:
> Based on the display spec documentation I would expect the overlay to render 
> fringes the same way as an inserted propertized string.

Thank you for your report.

This was never supported in Emacs.  The reasons are subtle and very
technical, and I will not go into them here.  Making this work as
expected might be possible, but it would require changes on a very low
level in the code that handles nested display properties and overlay
strings, which is already extremely complicated.  I find changes of
such nature unjustified for such a fringe (pun intended) use case.

As an easy work-around, you can have 2 different overlay strings at
the same position, like this:

  (defun insert-two-overlays ()
    (interactive)
    (remove-overlays)
    (let ((ov1 (make-overlay (point) (point)))
          (ov2 (make-overlay (point) (point))))
      (overlay-put ov1 'before-string
                   (propertize " " 'display '(left-fringe hollow-square)))
      (overlay-put ov2 'before-string
                   (propertize " " 'display '(right-fringe hollow-square)))))