bug#70697: 30.0.50; Frame parameter alpha-background is ignored for fringe bitmaps & internal borders

[Eli Zaretskii <eliz@gnu.org>]

> From: Aleksandar Dimitrov <mail@aleks.bg>
> Date: Wed, 01 May 2024 12:52:58 +0200
> 
> Hi all,
> 
> Runinng in any compositing window manager (this includes Wayland with a
> PGTK build of Emacs, but also X without PGTK), the frame parameter
> alpha-background is ignored for at least
> 
> - fringe bitmaps
> - insets drawn with internal-border-width
> 
> The resulting visual effect is quite jarring, and makes both of these
> difficult to use together with transparency.
> 
> Here's a small emacs -Q script to exhibit the problem:
> 
> #+begin_src emacs-lisp
> 
> ;; Turn off scroll bars to make the bugs more visible.
> (scroll-bar-mode -1)
> 
> ;; set transparency & internal border width.
> ;; Note how the internal border is going to be drawn in a solid white colour
> ;; and will ignore the alpha value.
> (modify-all-frames-parameters '((alpha-background . 50)
>                                 (internal-border-width . 20)))
> 
> (define-fringe-bitmap 'test-bitmap
>   (vector (expt 2 15)) nil 16 '(top t))
> 
> ;; Define a face for the fringe that has a different background colour, to
> ;; more easily discern the fringe bitmap bug from the internal border bug.
> (make-face 'test-face)
> (set-face-attribute 'test-face nil :background "pink")
> 
> ;; Apply a fringe bitmap to all lines in the current buffer. Note how the fringe
> ;; bitmap is monochrome, but the background colour ignores the global alpha value.
> (defun apply-fringe-bitmap ()
>   "Apply a fringe bitmap to the current buffer to exhibit the transparency bug."
>   (add-text-properties
>    (point-min) (point-max)
>    '(wrap-prefix
>      #(" " 0 1 (display (left-fringe test-bitmap test-face)))
>      line-prefix
>      #(" " 0 1 (display (left-fringe test-bitmap test-face))))))
> 
> (add-hook 'emacs-startup-hook #'apply-fringe-bitmap)
> 
> #+end_src
> 
> 
> And here's how it looks like: https://aleks.bg/emacs-bug.png
> (that's my personal home page, not an image hoster.)
> 
> I would expect the white border around the frame to also be transparent,
> and the fringe, too.

Po Lu, any comments or ideas about this?

Thanks.