docs: keep the org-element cache reproduction evidenceHEADmain

Twelve scripts plus a README covering the persist-write cache corruption:
the no-dirvish root cause, the buffer-list ordering dependency, a faithful
replay of the file-manager preview path, and the two fixes I tried and
reverted.

The README says which scripts are weak evidence and why, so the next reader
doesn't rebuild an argument on the one whose control varied three things at
once.

Also holds the upstream bug report and its reproduction, verified to run
standalone under emacs -Q.

1 files changed, 88 insertions, 0 deletions

diff --git a/working/org-element-cache-persist-bug/repro-preview-cache.el b/working/org-element-cache-persist-bug/repro-preview-cache.el
new file mode 100644
index 00000000..2a527af1
--- /dev/null
+++ b/working/org-element-cache-persist-bug/repro-preview-cache.el
@@ -0,0 +1,88 @@
+;;; repro-preview-cache.el --- Does dirvish's text preview disturb an org buffer's element cache? -*- lexical-binding: t -*-
+
+;; Replays `dirvish--preview-file-maybe-truncate' (dirvish.el:653-684) against an
+;; org file that is ALSO open for real, then kills the preview buffer the way
+;; dirvish does (`dirvish--kill-buffer', dirvish.el:416).  The kill is the step
+;; that matters: killing an org-mode buffer with a `buffer-file-name' is when
+;; org-persist writes that file's cache.
+
+(require 'org)
+(require 'org-element)
+(require 'org-persist)
+
+(setq org-element-use-cache t
+      org-element-cache-persistent t
+      org-element--cache-self-verify t
+      org-element--cache-self-verify-frequency 1.0)
+
+(defun repro--build-cache (buf)
+  "Actually populate the element cache of BUF by walking it."
+  (with-current-buffer buf
+    (org-element-cache-map (lambda (_el) nil) :granularity 'element)
+    (and org-element--cache (avl-tree-size org-element--cache))))
+
+(defun repro--warnings ()
+  (when (get-buffer "*Warnings*")
+    (with-current-buffer "*Warnings*" (string-trim (buffer-string)))))
+
+(let* ((src (expand-file-name "todo.org" default-directory))
+       (file (make-temp-file "repro-" nil ".org"))
+       (threshold 1048576))
+  (copy-file src file t)
+  (let ((size (nth 7 (file-attributes file))))
+    (message "== fixture %s (%d bytes, %s threshold) =="
+             (file-name-nondirectory file) size
+             (if (>= size threshold) "over" "under")))
+
+  (let ((real (find-file-noselect file)))
+    (message "real buffer cache after full walk: %S" (repro--build-cache real))
+
+    ;; --- dirvish text preview of the same file ---
+    (with-current-buffer (get-buffer-create "*preview-temp*")
+      (let (jka-compr-verbose)
+        (with-silent-modifications
+          (setq buffer-read-only t)
+          (insert-file-contents file nil 0 threshold)
+          (when (>= (nth 7 (file-attributes file)) threshold)
+            (goto-char (point-max))
+            (insert "\n\nFile truncated. End of partial preview.\n"))
+          (setq buffer-file-name file)
+          (goto-char (point-min))
+          (rename-buffer (format "PREVIEW :: %s" (file-name-nondirectory file))))
+        (let ((enable-dir-local-variables nil)
+              (enable-local-variables :safe)
+              (non-essential t))
+          (set-auto-mode)
+          (font-lock-mode 1)))
+      (message "preview buffer: mode=%s claims=%s cache=%S"
+               major-mode (file-name-nondirectory (or buffer-file-name "none"))
+               (repro--build-cache (current-buffer))))
+
+    (message "-- both buffers live: real=%S claiming-file=%d --"
+             (with-current-buffer real (avl-tree-size org-element--cache))
+             (length (seq-filter (lambda (b) (equal (buffer-local-value 'buffer-file-name b) file))
+                                 (buffer-list))))
+
+    ;; --- dirvish kills the preview buffer when the session ends ---
+    (let ((kill-buffer-query-functions nil)
+          (preview (format "PREVIEW :: %s" (file-name-nondirectory file))))
+      (kill-buffer preview)
+      (message "preview buffer %S killed" preview))
+
+    ;; --- is the real buffer still coherent? ---
+    (with-current-buffer real
+      ;; Edit it the way Craig would after stepping back from dirvish.
+      (goto-char (point-max))
+      (condition-case err
+          (let ((inhibit-read-only t)) (insert "\n* Repro edit\n"))
+        (error (message "RESULT: insert into real buffer SIGNALLED %S" (car err))))
+      (condition-case err
+          (progn (org-element-cache-map (lambda (_el) nil) :granularity 'element)
+                 (message "post-kill cache-map: clean (size=%S)"
+                          (avl-tree-size org-element--cache)))
+        (error (message "post-kill cache-map: ERROR %S" err)))
+      (set-buffer-modified-p nil))
+
+    (message "warnings=%s" (or (repro--warnings) "none")))
+
+  (delete-file file))