fix(ai-vterm): preserve single-window layout across F9 toggle

When the agent buffer is the only window in the frame, F9 buries it (correct) but the next F9 redisplayed it as a side split instead of restoring the full-frame layout -- the display-saved path always called `display-buffer-in-direction`, which insists on a split.

New `cj/--ai-vterm-last-was-bury` flag tracks which toggle-off path ran. `cj/ai-vterm` sets it to t in the bury branch (one-window-p) and nil in the delete-window branch. `cj/--ai-vterm-display-saved` checks the flag at toggle-on: if t and the frame is still single-window, it replaces the selected window's buffer in place rather than splitting. Either branch consumes the flag so it never stays stale.

5 tests in test-ai-vterm--single-window-toggle.el cover the flag's set/clear paths, the still-one-window guard, and the end-to-end roundtrip.

3 files changed, 224 insertions, 23 deletions

diff --git a/modules/ai-vterm.el b/modules/ai-vterm.el
index ac246ee8..1eae19f3 100644
--- a/modules/ai-vterm.el
+++ b/modules/ai-vterm.el
@@ -337,6 +337,16 @@ has been toggled off yet this session, so the default direction
 applies.  Captured at toggle-off by `cj/--ai-vterm-capture-state'
 and consumed by `cj/--ai-vterm-display-saved'.")
 
+(defvar cj/--ai-vterm-last-was-bury nil
+  "Non-nil when the last F9 toggle-off used `bury-buffer'.
+
+Set by `cj/ai-vterm' in its `toggle-off' branch: t when the agent
+window was the only window in the frame (so toggle-off buried
+without deleting), nil when the window was deleted.  Consumed by
+`cj/--ai-vterm-display-saved' to decide between restoring the
+buried agent in the current window (the only one) or splitting per
+the saved direction.")
+
 (defvar cj/--ai-vterm-last-size nil
   "Last user-chosen body size for the AI-vterm display.
 
@@ -398,12 +408,26 @@ project changes."
 (defun cj/--ai-vterm-display-saved (buffer alist)
   "Display-buffer action: split per saved direction and size.
 
-Delegates to `cj/window-toggle-display-saved' against the F9 state
-vars, falling back to `right' and `cj/ai-vterm-window-width'."
-  (cj/window-toggle-display-saved
-   buffer alist
-   'cj/--ai-vterm-last-direction 'right
-   'cj/--ai-vterm-last-size cj/ai-vterm-window-width))
+When the prior toggle-off was a bury (single-window state, flagged
+via `cj/--ai-vterm-last-was-bury') and the frame is still single-
+window, restore the agent into the selected window in place rather
+than splitting -- preserves the user's lone-window layout across
+F9 toggles.
+
+Otherwise delegates to `cj/window-toggle-display-saved' against the
+F9 state vars, falling back to `right' and `cj/ai-vterm-window-width'."
+  (cond
+   ((and cj/--ai-vterm-last-was-bury (one-window-p))
+    (setq cj/--ai-vterm-last-was-bury nil)
+    (let ((win (selected-window)))
+      (set-window-buffer win buffer)
+      win))
+   (t
+    (setq cj/--ai-vterm-last-was-bury nil)
+    (cj/window-toggle-display-saved
+     buffer alist
+     'cj/--ai-vterm-last-direction 'right
+     'cj/--ai-vterm-last-size cj/ai-vterm-window-width))))
 
 (defun cj/--ai-vterm-display-rule-list ()
   "Return the `display-buffer-alist' entry list installed by this module.
@@ -679,10 +703,17 @@ AI-vterm buffers without touching the project list."
      ;; other buffer in it, and the next toggle-on then creates a
      ;; fresh side window for a count of N+1.  Skip the deletion
      ;; only when agent is the lone window in the frame (delete
-     ;; would leave none); bury in that case.
-     (if (one-window-p)
-         (bury-buffer (window-buffer win))
-       (delete-window win))
+     ;; would leave none); bury in that case.  The flag tells the
+     ;; next toggle-on (via `cj/--ai-vterm-display-saved') to restore
+     ;; in place rather than splitting -- preserves the single-window
+     ;; layout across F9 toggles.
+     (cond
+      ((one-window-p)
+       (setq cj/--ai-vterm-last-was-bury t)
+       (bury-buffer (window-buffer win)))
+      (t
+       (setq cj/--ai-vterm-last-was-bury nil)
+       (delete-window win)))
      nil)
     (`(redisplay-recent . ,buf)
      (display-buffer buf)
diff --git a/tests/test-ai-vterm--single-window-toggle.el b/tests/test-ai-vterm--single-window-toggle.el
new file mode 100644
index 00000000..85108a01
--- /dev/null
+++ b/tests/test-ai-vterm--single-window-toggle.el
@@ -0,0 +1,170 @@
+;;; test-ai-vterm--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-vterm-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-vterm)
+(require 'testutil-vterm-buffers)
+
+;;; Normal Cases
+
+(ert-deftest test-ai-vterm--single-window-toggle-normal-roundtrip-preserves-fullscreen ()
+  "Normal: agent in the only window, simulated bury, F9 (on) -> still single window with agent.
+
+Reproduces Craig's report.  Before the 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.
+
+The bury step is simulated (set flag + swap window buffer to a non-
+agent buffer) because batch-mode `bury-buffer' won't switch the
+displayed buffer on a window with empty prev-buffers; the toggle-off
+branch's *logic* is covered by the flag-set-on-bury test."
+  (cj/test--kill-agent-buffers)
+  (let ((agent-name "agent [single-window-roundtrip]")
+        (other-name "*test-sw-roundtrip-other*")
+        (cj/--ai-vterm-last-was-bury nil)
+        (cj/--ai-vterm-last-direction nil)
+        (cj/--ai-vterm-last-size nil))
+    (unwind-protect
+        (save-window-excursion
+          (delete-other-windows)
+          (let ((agent-buf (get-buffer-create agent-name))
+                (other-buf (get-buffer-create other-name)))
+            (set-window-buffer (selected-window) agent-buf)
+            (should (one-window-p))
+            ;; Simulate the toggle-off bury path: capture state, set the
+            ;; bury flag, and put a non-agent buffer in the window where
+            ;; the real bury would have left one.  This isolates the
+            ;; toggle-on behaviour without depending on batch-mode
+            ;; `bury-buffer' (which is unreliable with empty prev-buffers).
+            (cj/--ai-vterm-capture-state (selected-window))
+            (setq cj/--ai-vterm-last-was-bury t)
+            (set-window-buffer (selected-window) other-buf)
+            (should (one-window-p))
+            (should-not (cj/--ai-vterm-displayed-agent-window))
+            ;; Toggle on -- should restore agent in the same lone window.
+            (let ((display-buffer-alist (cj/--ai-vterm-display-rule-list)))
+              (cj/ai-vterm))
+            (should (one-window-p))
+            (let ((win (cj/--ai-vterm-displayed-agent-window)))
+              (should (windowp win))
+              (should (eq (window-buffer win) agent-buf)))
+            ;; Flag must be consumed by the display-saved action.
+            (should-not cj/--ai-vterm-last-was-bury)))
+      (when (get-buffer other-name) (kill-buffer other-name))
+      (cj/test--kill-agent-buffers))))
+
+(ert-deftest test-ai-vterm--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-vterm-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-vterm-display-rule-list)))
+              (cj/ai-vterm)
+              (should (eq cj/--ai-vterm-last-was-bury t)))))
+      (cj/test--kill-agent-buffers))))
+
+(ert-deftest test-ai-vterm--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-vterm-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-vterm-display-rule-list)))
+              (set-window-buffer agent-win agent-buf)
+              (select-window agent-win)
+              (cj/ai-vterm)
+              (should-not cj/--ai-vterm-last-was-bury))))
+      (when (get-buffer left-name) (kill-buffer left-name))
+      (cj/test--kill-agent-buffers))))
+
+;;; Boundary Cases
+
+(ert-deftest test-ai-vterm--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-vterm-last-was-bury t)        ; flag pretends prior bury
+        (cj/--ai-vterm-last-direction 'right)
+        (cj/--ai-vterm-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-vterm-display-rule-list)))
+              (cl-letf (((symbol-function 'display-buffer-in-direction)
+                         (lambda (b _a)
+                           (setq received-buf b)
+                           (selected-window))))
+                (cj/--ai-vterm-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-vterm-last-was-bury))))
+      (when (get-buffer "*test-sw-other*") (kill-buffer "*test-sw-other*"))
+      (cj/test--kill-agent-buffers))))
+
+(ert-deftest test-ai-vterm--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-vterm-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-vterm-display-rule-list)))
+              (set-window-buffer agent-win agent-buf)
+              (select-window agent-win)
+              (cj/ai-vterm)
+              (should-not cj/--ai-vterm-last-was-bury))))
+      (when (get-buffer "*test-sw-untouched-left*")
+        (kill-buffer "*test-sw-untouched-left*"))
+      (cj/test--kill-agent-buffers))))
+
+(provide 'test-ai-vterm--single-window-toggle)
+;;; test-ai-vterm--single-window-toggle.el ends here
diff --git a/todo.org b/todo.org
index f5a07001..97a77094 100644
--- a/todo.org
+++ b/todo.org
@@ -122,25 +122,25 @@ attachment via =transient-append-suffix=, or gptel-side backend/model config.
 Migration to current lazy form: commit =3eb1a0c refactor(gptel): lazy-load
 gptel-magit, rebind rewrite/context keys=.
 
-** TODO [#B] F9 toggle should restore single-window layout for AI-vterm :bug:
+** DONE [#B] F9 toggle should restore single-window layout for AI-vterm :bug:
 
 When the AI-vterm buffer is the only window in the frame (e.g. after =C-x 1=) and
 F9 is pressed, =cj/ai-vterm= buries the buffer (correct), but the next F9
 redisplays the agent in a split rather than restoring the single-window full-frame
 layout. F9 should toggle off/on while preserving the lone-window state.
 
-The bury-then-redisplay path goes through =display-buffer= and hits
-=display-buffer-in-direction= via =cj/--ai-vterm-display-saved=, which splits the
-existing window. The fix needs the toggle-on path to detect that toggle-off was a
-bury-not-delete and use =switch-to-buffer= (or equivalent) instead of a
-saved-direction split.
-
-Locations:
-- =modules/ai-vterm.el= around the =(one-window-p)= branch in =cj/ai-vterm=
-- =modules/ai-vterm.el= the =cj/--ai-vterm-display-saved= display-buffer action
-
-Regression test: enter single-window state with agent buffer, F9, F9 again --
-expect =(one-window-p)= still t and the agent buffer displayed.
+Fix: new =cj/--ai-vterm-last-was-bury= flag in =modules/ai-vterm.el=.
+The toggle-off branch sets it to t when =one-window-p= is true (bury
+path) or nil when =delete-window= runs.  =cj/--ai-vterm-display-saved=
+checks the flag at toggle-on: if t and the frame is still single-window,
+it replaces the selected window's buffer in place via =set-window-buffer=
+rather than splitting via =display-buffer-in-direction=.  Flag is
+consumed (cleared) by either branch so it never stays stale.
+
+5 tests in =tests/test-ai-vterm--single-window-toggle.el= cover:
+flag set on bury, flag cleared on delete-window, flag respected only
+when still one-window, flag not set when bury didn't run, and the
+end-to-end roundtrip.  Full =make test-unit= green.
 
 ** TODO [#B] AI-vterm scrollback history should replace agent buffer in place :feature: