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+;;; test-system-defaults--warning-display-dead-buffer.el --- Dead-buffer guard on deferred warnings -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Emacs 31.1's warnings.el defers daemon-startup warnings into a one-shot
+;; `after-make-frame-functions' closure that calls `warning--display-buffer'
+;; on the first client frame with the *Warnings* buffer object it captured at
+;; warning time.  If anything killed that buffer in between, `display-buffer'
+;; signals inside `make-frame', server.el swallows the error as
+;; "-window-system-unsupported", and emacsclient silently retries on $DISPLAY:
+;; the session's first frame lands on XWayland.
+;;
+;; The root fix keeps *Warnings* alive (undead-buffers.el).  This is the
+;; defense in depth: `cj/warning--display-buffer-if-live' wraps
+;; `warning--display-buffer' so a dead buffer is skipped rather than passed
+;; on.  Load happens once in the shared sandbox (testutil-system-defaults.el);
+;; `warning--display-buffer' only exists from Emacs 31, so the end-to-end case
+;; skips on older builds while the pure-function cases always run.
+
+;;; Code:
+
+(require 'ert)
+(add-to-list 'load-path (expand-file-name "tests" user-emacs-directory))
+(require 'testutil-system-defaults)
+
+(test-system-defaults--with-load-environment
+  (test-system-defaults--load))
+
+(defun test-system-defaults--recording-orig ()
+  "Return (ORIG . CALLS) where ORIG records every argument into CALLS."
+  (let ((calls (list nil)))
+    (cons (lambda (buffer)
+            (push buffer (car calls))
+            'displayed)
+          calls)))
+
+;;; Normal Cases
+
+(ert-deftest test-system-defaults-warning-guard-passes-live-buffer-through ()
+  "Normal: a live buffer reaches the original and its value is returned."
+  (let* ((rec (test-system-defaults--recording-orig))
+         (buf (generate-new-buffer " *warning-guard-live*")))
+    (unwind-protect
+        (progn
+          (should (eq 'displayed
+                      (cj/warning--display-buffer-if-live (car rec) buf)))
+          (should (equal (list buf) (car (cdr rec)))))
+      (kill-buffer buf))))
+
+(ert-deftest test-system-defaults-warning-guard-is-installed ()
+  "Normal: loading system-defaults installs the guard on the deferred display.
+From Emacs 31 the advised symbol must actually be defined: pending advice on
+an undefined symbol would still count as installed, so a rename upstream
+would otherwise silently disable the backstop."
+  (should (advice-member-p #'cj/warning--display-buffer-if-live
+                           'warning--display-buffer))
+  (when (>= emacs-major-version 31)
+    (should (fboundp 'warning--display-buffer))))
+
+(ert-deftest test-system-defaults-warning-guard-resolves-live-buffer-name ()
+  "Normal: a live buffer's name is resolved and passed through as the buffer."
+  (let* ((rec (test-system-defaults--recording-orig))
+         (buf (generate-new-buffer " *warning-guard-named*")))
+    (unwind-protect
+        (progn
+          (should (eq 'displayed
+                      (cj/warning--display-buffer-if-live
+                       (car rec) (buffer-name buf))))
+          (should (equal (list buf) (car (cdr rec)))))
+      (kill-buffer buf))))
+
+;;; Boundary Cases
+
+(ert-deftest test-system-defaults-warning-guard-skips-killed-buffer ()
+  "Boundary: a killed buffer never reaches the original; result is nil."
+  (let* ((rec (test-system-defaults--recording-orig))
+         (buf (generate-new-buffer " *warning-guard-dead*")))
+    (kill-buffer buf)
+    (should-not (cj/warning--display-buffer-if-live (car rec) buf))
+    (should-not (car (cdr rec)))))
+
+(ert-deftest test-system-defaults-warning-guard-end-to-end-dead-buffer-does-not-signal ()
+  "Boundary: the real deferred display survives a dead buffer.
+Mirrors the live failure: a string condition in `display-buffer-alist' is
+what `buffer-match-p' tripped over when the buffer name came back nil."
+  (skip-unless (fboundp 'warning--display-buffer))
+  (let ((buf (generate-new-buffer "*Warnings*"))
+        (display-buffer-alist '(("^ \\*test-guard\\*" display-buffer-no-window))))
+    (kill-buffer buf)
+    (should-not (warning--display-buffer buf))))
+
+;;; Error Cases
+
+(ert-deftest test-system-defaults-warning-guard-rejects-non-buffer ()
+  "Error: nil, or a name that resolves to no buffer, is skipped without a signal."
+  (let ((rec (test-system-defaults--recording-orig))
+        (missing " *warning-guard-no-such-buffer*"))
+    (when (get-buffer missing) (kill-buffer missing))
+    (should-not (cj/warning--display-buffer-if-live (car rec) nil))
+    (should-not (cj/warning--display-buffer-if-live (car rec) missing))
+    (should-not (car (cdr rec)))))
+
+(provide 'test-system-defaults--warning-display-dead-buffer)
+;;; test-system-defaults--warning-display-dead-buffer.el ends here