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@jdtsmith
Last active December 6, 2023 18:16
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bug67604
;; 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")))),
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jdtsmith commented Dec 6, 2023

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

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