repro-719.el

;;; repro-719.el --- Minimal reproducer for agent-shell #719 -*- lexical-binding: t; -*-

;; Maybe isolates the hang reported in
;; https://github.com/xenodium/agent-shell/issues/719 down to a single
;; overlay whose `before-string'/`display' strings contain embedded
;; newlines.  No agent-shell (or any package) required.
;;
;; Run in a THROWAWAY Emacs -- this may hang the session:
;;
;;   emacs -Q -l repro-719.el
;;
;; Two commands are provided:
;;
;;   M-x repro-719-buggy  -- overlay with newline-laden display strings,
;;                           mirroring agent-shell's "changes" hunk box.
;;   M-x repro-719-safe   -- same label rendered without embedded newlines.
;;
;; In each buffer point starts on the overlay line near the top.  Press
;; <up> (previous-line) a few times, or scroll up.  On the affected
;; builds (seen on Emacs 31), `repro-719-buggy' hangs in
;; `move_it_vertically_backward'; `repro-719-safe' does not.  Use C-x C-c
;; to quit, or kill the process if it hangs.

;;; Code:

(defun repro-719--make-buffer (name setup)
  "Create buffer NAME with filler text and run SETUP on the @@ line.
SETUP receives the overlay covering the `@@' line."
  (let ((buffer (get-buffer-create name)))
    (with-current-buffer buffer
      (erase-buffer)
      ;; A couple of lines above so there is somewhere to scroll up into.
      (insert "top of buffer\n")
      (insert "context line\n")
      (let ((hunk-start (point)))
        (insert "@@ -1,4 +1,4 @@\n")
        (dotimes (i 400)
          (insert (format " unchanged line %d\n" i)))
        (goto-char hunk-start)
        (let ((overlay (make-overlay (line-beginning-position)
                                     (line-end-position))))
          (funcall setup overlay)))
      (setq buffer-read-only t)
      (goto-char (point-min))
      (forward-line 2)) ; land on the overlay line
    (pop-to-buffer buffer)
    (message "Press <up> a few times (or scroll up). See top of repro-719.el.")))

(defun repro-719-buggy ()
  "Reproduce the hang: overlay strings with embedded newlines."
  (interactive)
  (repro-719--make-buffer
   "*repro-719-buggy*"
   (lambda (overlay)
     (overlay-put overlay 'before-string
                  (propertize "\n╭─────────╮\n" 'face 'diff-hunk-header))
     (overlay-put overlay 'display
                  (propertize "│ changes │\n╰─────────╯\n\n"
                              'face 'diff-hunk-header)))))

(defun repro-719-safe ()
  "Control: same label, single line, no embedded newlines."
  (interactive)
  (repro-719--make-buffer
   "*repro-719-safe*"
   (lambda (overlay)
     (overlay-put overlay 'display
                  (propertize " changes "
                              'face '(:inherit diff-hunk-header :box t))))))

(repro-719-buggy)

(provide 'repro-719)

;;; repro-719.el ends here