feat(vterm): show tmux scrollback history in place

`cj/vterm-tmux-history' previously used `pop-to-buffer', which routed the history view through display-buffer-alist -- in an agent window that often meant a split or a hand-off to another window, costing the agent its frame slot. `switch-to-buffer' instead drops the history into the selected window directly; the existing quit handler already restores the origin in that same window via `set-window-buffer'.

New test asserts the in-place behavior: starting single-window with a vterm origin, invoking the command leaves `(one-window-p)` t with the history buffer in the original slot. The existing render test no longer needs its `pop-to-buffer' stub since `switch-to-buffer' works in batch.

3 files changed, 71 insertions, 21 deletions

diff --git a/modules/vterm-config.el b/modules/vterm-config.el
index 764c9dcd..954e096a 100644
--- a/modules/vterm-config.el
+++ b/modules/vterm-config.el
@@ -121,7 +121,12 @@ returns to the vterm without copying.  RET is left unbound."
 The history buffer uses normal Emacs navigation and selection.  `M-w'
 copies the active region and stays open, so several pieces can be
 copied in a row; `q', `<escape>', or `C-g' returns point to the vterm
-buffer that launched it."
+buffer that launched it.
+
+The history view replaces the origin vterm buffer in the same window
+(via `switch-to-buffer'), not a split or a popped-up window -- reading
+past output should keep the agent's frame slot intact, and quit puts
+the live terminal back where it was."
   (interactive)
   (let* ((origin-buffer (current-buffer))
          (origin-window (selected-window))
@@ -139,7 +144,7 @@ buffer that launched it."
       (setq-local cj/vterm-tmux-history--origin-window origin-window)
       (setq-local cj/vterm-tmux-history--origin-point origin-point)
       (goto-char (point-max)))
-    (pop-to-buffer buffer)))
+    (switch-to-buffer buffer)))
 
 (defun cj/vterm-copy-mode-cancel ()
   "Exit `vterm-copy-mode' without copying."
diff --git a/tests/test-vterm-tmux-history.el b/tests/test-vterm-tmux-history.el
index eec6c622..c0a71421 100644
--- a/tests/test-vterm-tmux-history.el
+++ b/tests/test-vterm-tmux-history.el
@@ -63,15 +63,12 @@ RESPONSES is an alist of (ARGS EXIT-CODE OUTPUT)."
   "Normal: command renders tmux history in a normal Emacs buffer."
   (let ((origin (cj/test--make-fake-vterm-buffer "*test-vterm-history-origin*")))
     (unwind-protect
-        (with-current-buffer origin
+        (save-window-excursion
+          (switch-to-buffer origin)
           (cl-letf (((symbol-function 'get-buffer-process)
                      (lambda (_buffer) 'fake-process))
                     ((symbol-function 'process-tty-name)
-                     (lambda (_process) "/dev/pts/8"))
-                    ((symbol-function 'pop-to-buffer)
-                     (lambda (buffer &rest _)
-                       (set-buffer buffer)
-                       buffer)))
+                     (lambda (_process) "/dev/pts/8")))
             (test-vterm-tmux-history--with-tmux-mock
                 '((("list-clients" "-F" "#{client_tty}\t#{pane_id}") 0
                    "/dev/pts/8\t%8\n")
@@ -81,10 +78,45 @@ RESPONSES is an alist of (ARGS EXIT-CODE OUTPUT)."
               (should (eq major-mode 'cj/vterm-tmux-history-mode))
               (should buffer-read-only)
               (should (string-match-p "history http://example.com"
-                                      (buffer-string)))))))
+                                      (buffer-string))))))
+      (cj/test--kill-buffers-matching-prefix "*vterm tmux history")
+      (when (buffer-live-p origin)
+        (kill-buffer origin)))))
+
+(ert-deftest test-vterm-tmux-history-replaces-origin-buffer-in-same-window ()
+  "Normal: the history view replaces the origin in the selected window.
+
+Before the in-place change, `cj/vterm-tmux-history' used `pop-to-buffer'
+which could split or hand the buffer to a different window.  The fix
+uses `switch-to-buffer' so reading scrollback keeps the agent's frame
+slot."
+  (let ((origin (cj/test--make-fake-vterm-buffer "*test-vterm-history-inplace*")))
+    (unwind-protect
+        (save-window-excursion
+          (delete-other-windows)
+          (switch-to-buffer origin)
+          (let ((win (selected-window)))
+            (should (eq (window-buffer win) origin))
+            (should (one-window-p))
+            (cl-letf (((symbol-function 'get-buffer-process)
+                       (lambda (_buffer) 'fake-process))
+                      ((symbol-function 'process-tty-name)
+                       (lambda (_process) "/dev/pts/8")))
+              (test-vterm-tmux-history--with-tmux-mock
+                  '((("list-clients" "-F" "#{client_tty}\t#{pane_id}") 0
+                     "/dev/pts/8\t%8\n")
+                    (("capture-pane" "-p" "-J" "-S" "-" "-E" "-" "-t" "%8") 0
+                     "scrollback line\n"))
+                (cj/vterm-tmux-history)))
+            ;; Same window, no split, history buffer now in the slot.
+            (should (one-window-p))
+            (should (eq (selected-window) win))
+            (should (string-prefix-p
+                     "*vterm tmux history:"
+                     (buffer-name (window-buffer win))))))
       (cj/test--kill-buffers-matching-prefix "*vterm tmux history")
       (when (buffer-live-p origin)
-        (kill-buffer origin))))
+        (kill-buffer origin)))))
 
 (ert-deftest test-vterm-tmux-history-quit-returns-to-origin ()
   "Normal: q / <escape> / C-g (cj/vterm-tmux-history-quit) kills the history
diff --git a/todo.org b/todo.org
index 97a77094..86fa4a2a 100644
--- a/todo.org
+++ b/todo.org
@@ -142,23 +142,36 @@ 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:
+** DONE [#B] AI-vterm scrollback history should replace agent buffer in place :feature:
 
 When viewing the scrollback history of an AI-vterm buffer, the history view should
 replace the live agent buffer in the same window rather than splitting or popping
 a separate window. Goal: read past output without losing the agent's frame slot,
 then snap back to the live buffer when done.
 
-Open questions before implementation:
-- Is the trigger an existing command (e.g. =vterm-copy-mode= toggle staying
-  in-place) or a new command that builds a read-only history buffer?
-- Round-trip ergonomics: how does the user return to the live agent? Same key as
-  the entry, or a separate "resume" binding?
-- Does this need to integrate with the F9 toggle state (so a toggle-off while in
-  history mode does the right thing)?
-
-Locations: =modules/ai-vterm.el= (no scrollback-history command exists yet) and
-=modules/vterm-config.el= (vterm base bindings).
+Decisions on the open questions:
+- *Trigger*: reused the existing =cj/vterm-tmux-history= command (=C-; x h=).
+  No new command -- it already captures the tmux pane and runs from any
+  vterm buffer including agents.
+- *Round-trip*: =q= / =<escape>= / =C-g= already restore the origin in
+  the same window via =cj/vterm-tmux-history-quit=.  Same key as the
+  scrollback mode's other exits.
+- *F9 integration*: deferred.  Pressing F9 in history mode now treats the
+  history buffer as non-agent (its name is =*vterm tmux history: ...*=,
+  not =agent [...]=) so dispatch falls through to redisplay-recent + a
+  saved-direction split.  A user who wants to toggle agent off should
+  press =q= first, then F9.  Filed as a follow-up if it bites.
+
+Fix: =modules/vterm-config.el= -- one line.  =pop-to-buffer buffer=
+became =switch-to-buffer buffer= so the history view replaces the origin
+in the selected window instead of going through display-buffer's split
+logic.  Quit was already in-place via =set-window-buffer=.
+
+New test in =tests/test-vterm-tmux-history.el= asserts the selected
+window's buffer becomes the history buffer with no extra window
+created (=one-window-p= still t).  Existing tests dropped their
+=pop-to-buffer= stub since =switch-to-buffer= works directly in batch.
+Full =make test-unit= green.
 
 ** TODO [#B] Add ERT coverage for modules below 70% :tests: