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+;;; test-ai-term--single-window-toggle.el --- F9 toggle round-trip when agent is the only window -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Regression coverage for the bug where toggling off a single-window
+;; agent (bury) then toggling on again redisplays the agent in a side
+;; split instead of restoring the full-frame layout.
+;;
+;; The fix introduces a `cj/--ai-term-last-was-bury' flag set at
+;; toggle-off when `one-window-p' was true.  At toggle-on the display
+;; action consumes the flag and, if the frame is still single-window,
+;; replaces the current window's buffer in place rather than calling
+;; `display-buffer-in-direction'.
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(add-to-list 'load-path (expand-file-name "tests" user-emacs-directory))
+(require 'ai-term)
+(require 'testutil-ghostel-buffers)
+
+;;; Normal Cases
+
+(ert-deftest test-ai-term--single-window-toggle-normal-roundtrip-preserves-fullscreen ()
+  "Normal: agent in the only window, F9 (off), F9 (on) -> still single window with agent.
+
+Reproduces Craig's report.  Before the original fix the toggle-on path
+fell through to `display-buffer-in-direction', which split the lone
+window into two and left the agent in a side panel.  Before the
+follow-up fix the toggle-off path could no-op entirely when
+`bury-buffer' couldn't find a buffer to switch to, so the user saw
+\"F9 does nothing\".  The dispatcher now forces the swap to a non-
+agent buffer after bury so the toggle-off is observable in real and
+batch use both."
+  (cj/test--kill-agent-buffers)
+  (let ((agent-name "agent [single-window-roundtrip]")
+        (cj/--ai-term-last-was-bury nil)
+        (cj/--ai-term-last-direction nil)
+        (cj/--ai-term-last-size nil))
+    (unwind-protect
+        (save-window-excursion
+          (delete-other-windows)
+          (let ((agent-buf (get-buffer-create agent-name)))
+            (set-window-buffer (selected-window) agent-buf)
+            (should (one-window-p))
+            (let ((display-buffer-alist (cj/--ai-term-display-rule-list)))
+              ;; Toggle off -- the dispatcher's force-swap should put the
+              ;; window on a non-agent buffer.
+              (cj/test--call-as-gui #'cj/ai-term)
+              (should (one-window-p))
+              (should-not (cj/--ai-term-displayed-agent-window))
+              (should (eq cj/--ai-term-last-was-bury t))
+              ;; Toggle on -- should restore agent in the same lone window.
+              (cj/test--call-as-gui #'cj/ai-term)
+              (should (one-window-p))
+              (let ((win (cj/--ai-term-displayed-agent-window)))
+                (should (windowp win))
+                (should (eq (window-buffer win) agent-buf)))
+              ;; Flag consumed by the display-saved action.
+              (should-not cj/--ai-term-last-was-bury))))
+      (cj/test--kill-agent-buffers))))
+
+(ert-deftest test-ai-term--single-window-toggle-off-swaps-window-buffer ()
+  "Normal: toggle-off in single-window state forces the window onto a non-
+agent buffer when `bury-buffer' itself didn't swap.
+
+Catches the regression Craig reported after the original fix shipped:
+F9 in a lone-window agent did nothing visible.  The fix layer here
+ensures the displayed buffer changes -- so the next F9 sees an empty
+agent-window state and can route through the display-saved path."
+  (cj/test--kill-agent-buffers)
+  (let ((agent-name "agent [bury-swap-observable]")
+        (cj/--ai-term-last-was-bury nil))
+    (unwind-protect
+        (save-window-excursion
+          (delete-other-windows)
+          (let* ((agent-buf (get-buffer-create agent-name))
+                 (win (selected-window)))
+            (set-window-buffer win agent-buf)
+            (let ((display-buffer-alist (cj/--ai-term-display-rule-list)))
+              (cj/test--call-as-gui #'cj/ai-term))
+            (should (window-live-p win))
+            (should-not (cj/--ai-term-buffer-p (window-buffer win)))))
+      (cj/test--kill-agent-buffers))))
+
+(ert-deftest test-ai-term--single-window-toggle-normal-flag-set-on-bury ()
+  "Normal: single-window toggle-off sets the bury flag."
+  (cj/test--kill-agent-buffers)
+  (let ((agent-name "agent [bury-flag-set]")
+        (cj/--ai-term-last-was-bury nil))
+    (unwind-protect
+        (save-window-excursion
+          (delete-other-windows)
+          (let ((agent-buf (get-buffer-create agent-name)))
+            (set-window-buffer (selected-window) agent-buf)
+            (let ((display-buffer-alist (cj/--ai-term-display-rule-list)))
+              (cj/test--call-as-gui #'cj/ai-term)
+              (should (eq cj/--ai-term-last-was-bury t)))))
+      (cj/test--kill-agent-buffers))))
+
+(ert-deftest test-ai-term--single-window-toggle-normal-flag-cleared-on-multi-window-off ()
+  "Normal: multi-window toggle-off clears the bury flag.
+Mirrors the existing `delete-window' branch of the dispatcher --
+the flag should not carry over a prior bury into a delete-window
+toggle-off."
+  (cj/test--kill-agent-buffers)
+  (let ((agent-name "agent [bury-flag-clear]")
+        (left-name "*test-sw-left*")
+        (cj/--ai-term-last-was-bury t))    ; stale t from prior bury
+    (unwind-protect
+        (save-window-excursion
+          (delete-other-windows)
+          (let ((agent-buf (get-buffer-create agent-name))
+                (left-buf (get-buffer-create left-name)))
+            (set-window-buffer (selected-window) left-buf)
+            (let* ((agent-win (split-window (selected-window) nil 'right))
+                   (display-buffer-alist (cj/--ai-term-display-rule-list)))
+              (set-window-buffer agent-win agent-buf)
+              (select-window agent-win)
+              (cj/test--call-as-gui #'cj/ai-term)
+              (should-not cj/--ai-term-last-was-bury))))
+      (when (get-buffer left-name) (kill-buffer left-name))
+      (cj/test--kill-agent-buffers))))
+
+;;; Boundary Cases
+
+(ert-deftest test-ai-term--single-window-toggle-boundary-flag-respected-only-when-still-one-window ()
+  "Boundary: if the frame got split between toggle-off and toggle-on, the
+saved-direction split applies as usual.  The flag is a fast-path for the
+genuine single-window case, not an override for every redisplay."
+  (cj/test--kill-agent-buffers)
+  (let ((agent-name "agent [flag-fallback]")
+        (cj/--ai-term-last-was-bury t)        ; flag pretends prior bury
+        (cj/--ai-term-last-direction 'right)
+        (cj/--ai-term-last-size 40))
+    (unwind-protect
+        (save-window-excursion
+          (delete-other-windows)
+          (let* ((other-buf (get-buffer-create "*test-sw-other*"))
+                 (agent-buf (get-buffer-create agent-name)))
+            (set-window-buffer (selected-window) other-buf)
+            ;; Frame is split (two windows) -- single-window precondition
+            ;; for the flag no longer holds.
+            (split-window-right)
+            (should-not (one-window-p))
+            (let (received-buf
+                  (display-buffer-alist (cj/--ai-term-display-rule-list)))
+              (cl-letf (((symbol-function 'display-buffer-in-direction)
+                         (lambda (b _a)
+                           (setq received-buf b)
+                           (selected-window))))
+                (cj/--ai-term-display-saved agent-buf nil))
+              ;; The saved-direction split path ran (display-buffer-in-direction
+              ;; was called) rather than the in-place fast path.
+              (should (eq received-buf agent-buf))
+              ;; And the flag is cleared either way.
+              (should-not cj/--ai-term-last-was-bury))))
+      (when (get-buffer "*test-sw-other*") (kill-buffer "*test-sw-other*"))
+      (cj/test--kill-agent-buffers))))
+
+(ert-deftest test-ai-term--single-window-toggle-boundary-flag-not-set-when-bury-not-used ()
+  "Boundary: a fresh dispatcher run with the agent displayed multi-window leaves
+the flag nil (no spurious set)."
+  (cj/test--kill-agent-buffers)
+  (let ((agent-name "agent [bury-flag-untouched]")
+        (cj/--ai-term-last-was-bury nil))
+    (unwind-protect
+        (save-window-excursion
+          (delete-other-windows)
+          (let ((agent-buf (get-buffer-create agent-name))
+                (left-buf (get-buffer-create "*test-sw-untouched-left*")))
+            (set-window-buffer (selected-window) left-buf)
+            (let* ((agent-win (split-window (selected-window) nil 'right))
+                   (display-buffer-alist (cj/--ai-term-display-rule-list)))
+              (set-window-buffer agent-win agent-buf)
+              (select-window agent-win)
+              (cj/test--call-as-gui #'cj/ai-term)
+              (should-not cj/--ai-term-last-was-bury))))
+      (when (get-buffer "*test-sw-untouched-left*")
+        (kill-buffer "*test-sw-untouched-left*"))
+      (cj/test--kill-agent-buffers))))
+
+(provide 'test-ai-term--single-window-toggle)
+;;; test-ai-term--single-window-toggle.el ends here