bug67604

inline-image-motion-bugs.el

;; Simple test exhibiting motion errors with inline images.
;; See Bug#67604
;; 1. Eval this buffer (M-x eval-buffer).
;; 2. A buffer with 3 colored SVG blocks will appear.
;; 3. Adjust your frame's width until the green image just wraps to
;;    the 2nd screen line, then reduce width by one more char.
;; 4. In the buffer with the images, M-x my/find-skip-bug.  If it
;;    succeeds, it will report the pixel offset where the bug
;;    occurred.  NOTE: The Green image must appear at the start of a
;;    line, or a "false positive" bug ID has occurred.  Increase your
;;    frame width one char and try again.
;; 5. Goto (point-min).  You'll notice that 2 next-line's put you on
;;    screen line 4: JUMPS HERE.
(require 'svg)
(let ((buf "svg-file-motion-demo")
      (ims '(("red" 146 29) ; 146 start
	     ("green" 108 29) ; 108 start
	     ("blue" 151 29))))
  (with-current-buffer (get-buffer-create buf)
    (erase-buffer)
    (visual-line-mode 1)
    (insert "tellus.  $\\gamma(t) = \\log\\left(\\sqrt{\\tan(t)}\\right)$  Donec hendrerit tempor tellus.  $\\chi(y) = \\sqrt{\\frac{1}{\\log(y)}}$  Phasellus lacus.  $\\tau(t) = \\exp\\left(\\sqrt{\\exp(t)}\\right)$  Curabitur lacinia pulvinar nibh.
JUMPS HERE")
    (goto-char (point-min))
    (while  (re-search-forward (rx ?$ (* (not ?$)) ?$) nil t)
      (let* ((ov (make-overlay (match-beginning 0) (match-end 0)))
	     (im (pop ims))
	     (svg (svg-create (nth 1 im) (nth 2 im))))
	(svg-rectangle svg 0 0 (nth 1 im) (nth 2 im) :fill-color (car im))
	(overlay-put ov 'display (svg-image svg :ascent 'center)))))
  (pop-to-buffer buf))

(defun my/find-skip-bug ()
  (interactive)
  (goto-char (point-min))
  (let* ((ov (car (overlays-at 10)))
	 (w 146)
	 (h 29)
	 (wc (frame-char-width))
	 (res
	  (cl-loop
	   for off from (- (- wc 2)) to (1- wc)
	   for sw = (+ w off)
	   for svg = (svg-create sw h) do
	   (message "Checking with red image width %d" sw)
	   (svg-rectangle svg 0 0 sw h :fill-color "red")
	   (overlay-put ov 'display (svg-image svg :ascent 'center))
	   if (save-excursion
		(next-line) (next-line)
		(beginning-of-visual-line)
		(looking-at "JUMPS HERE"))
	   return off
	   finally return nil)))
    (if res (message "Found Bug at offset %d = %d pixels" res (+ w res))
      (message "Did not find Bug")))),

This is a video of the bug being discovered on a Linux system with master. First, the test code above is eval'd in the *scratch* buffer, then various frame-widths are tried until a real version of the bug is found.

Screen.Recording.2023-12-06.at.1.10.56.PM.mov