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

[Eli Zaretskii <eliz@gnu.org>]

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)))))