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

From: Aleksandar Dimitrov <[email-masked]>
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.

Aleks