feat(term): add cj/term-tmux-detach on C-; x d

A keyboard C-b inside the Claude Code pane never reaches tmux as a prefix. It lands as stray text in the TUI's input, so C-b d can't detach the client. The new command writes C-b d into the pty instead, the same path cj/term-copy-mode-dwim already uses for C-b [. It sits on cj/term-map "d" next to copy-mode on "c", and outside tmux it writes nothing and says so.

2 files changed, 81 insertions, 2 deletions

diff --git a/modules/eat-config.el b/modules/eat-config.el
index 01d0fbe6..f764848e 100644
--- a/modules/eat-config.el
+++ b/modules/eat-config.el
@@ -415,7 +415,8 @@ terminal.  ai-term's agent buffers are managed separately via M-SPC."
 ;; Carried over from the ghostel era for the EAT agent terminals (ai-term).
 ;; Agents run EAT over tmux, so copy-mode is tmux's own copy-mode -- the same UX
 ;; ghostel-over-tmux had.  C-<up> enters it and scrolls up in one stroke; C-; x c
-;; enters it via the menu, and C-; x h grabs the whole pane history into a buffer.
+;; enters it via the menu, C-; x h grabs the whole pane history into a buffer,
+;; and C-; x d detaches the tmux client without going through its prefix.
 
 (declare-function cj/register-prefix-map "keybindings")
 (declare-function eat-emacs-mode "eat")
@@ -585,6 +586,20 @@ scrollback) and moves point to the start of the line."
     (eat-emacs-mode)
     (beginning-of-line)))
 
+(defun cj/term-tmux-detach ()
+  "Detach the tmux client from inside an agent terminal.
+Writes tmux's prefix and the detach key (C-b d) straight into the pty, the
+same path `cj/term-copy-mode-dwim' uses for C-b [.  A keyboard C-b inside
+the Claude Code pane has been observed to land as stray text instead of
+reaching tmux as a prefix (root cause not yet pinned down), so the string
+path is the reliable one.  Outside tmux it writes
+nothing and says so, since C-b d typed into a plain shell is just a control
+character."
+  (interactive)
+  (if (cj/term--in-tmux-p)
+      (cj/--term-send-string "\C-bd")
+    (message "cj/term-tmux-detach: not attached to tmux")))
+
 (defun cj/term--tmux-pane-in-copy-mode-p (pane-id)
   "Return non-nil when tmux PANE-ID is currently displaying a mode.
 tmux's `pane_in_mode' is 1 while a pane is in any mode; copy-mode is the only
@@ -613,13 +628,15 @@ pty; without tmux, moves point up in EAT's emacs-mode buffer."
         (cj/term-copy-mode-dwim))
       (forward-line -1)))))
 
-;; The C-; x terminal prefix (copy-mode, tmux history, the F12 toggle).  C-<up>
+;; The C-; x terminal prefix (copy-mode, tmux detach, tmux history, the F12
+;; toggle).  C-<up>
 ;; enters copy-mode + scrolls in one stroke; bound in EAT's semi-char map so it
 ;; reaches Emacs from inside an agent terminal.
 (defvar-keymap cj/term-map
   :doc "Personal terminal command map.")
 (cj/register-prefix-map "x" cj/term-map)
 (keymap-set cj/term-map "c" #'cj/term-copy-mode-dwim)
+(keymap-set cj/term-map "d" #'cj/term-tmux-detach)
 (keymap-set cj/term-map "h" #'cj/term-tmux-history)
 (keymap-set cj/term-map "t" #'cj/term-toggle)
 
diff --git a/tests/test-term-tmux-detach.el b/tests/test-term-tmux-detach.el
new file mode 100644
index 00000000..9bd94776
--- /dev/null
+++ b/tests/test-term-tmux-detach.el
@@ -0,0 +1,62 @@
+;;; test-term-tmux-detach.el --- Tests for cj/term-tmux-detach -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; A keyboard C-b inside the Claude Code pane does not reach tmux as a prefix
+;; (it lands as stray text), so detaching needs the same pty string path
+;; `cj/term-copy-mode-dwim' uses for C-b [.  These tests pin that path and
+;; the no-tmux fallback.
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+(require 'package)
+
+;; Same shape as test-term-tmux-history.el: `make test' runs with no
+;; package-initialize, so eat has to be made loadable here before eat-config.
+(setq package-user-dir (expand-file-name "elpa" user-emacs-directory))
+(package-initialize)
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(add-to-list 'load-path (expand-file-name "tests" user-emacs-directory))
+(setq load-prefer-newer t)
+(require 'eat)
+(require 'eat-config)
+
+(ert-deftest test-eat-config-tmux-detach-sends-prefix-and-d-when-attached ()
+  "Normal: with tmux attached, the command writes C-b d into the pty, nothing else."
+  (let ((sent nil))
+    (cl-letf (((symbol-function 'cj/term--in-tmux-p) (lambda () t))
+              ((symbol-function 'cj/--term-send-string) (lambda (s) (push s sent))))
+      (cj/term-tmux-detach)
+      (should (equal sent '("\C-bd"))))))
+
+(ert-deftest test-eat-config-tmux-detach-does-nothing-without-tmux ()
+  "Boundary: with no tmux client, nothing is written and the user is told why.
+Writing C-b d into a plain shell would type a control character into it."
+  (let ((sent nil)
+        (told nil))
+    (cl-letf (((symbol-function 'cj/term--in-tmux-p) (lambda () nil))
+              ((symbol-function 'cj/--term-send-string) (lambda (s) (push s sent)))
+              ((symbol-function 'message) (lambda (fmt &rest args)
+                                            (setq told (apply #'format fmt args)))))
+      (cj/term-tmux-detach)
+      (should-not sent)
+      (should (string-match-p "tmux" told)))))
+
+(ert-deftest test-eat-config-tmux-detach-survives-dead-process ()
+  "Error: with tmux reported attached but no live pty, the command returns
+without signalling.  `cj/--term-send-string' already guards on
+`process-live-p'; this pins that the detach path relies on it rather than
+calling `process-send-string' directly."
+  (with-temp-buffer
+    (cl-letf (((symbol-function 'cj/term--in-tmux-p) (lambda () t)))
+      (should-not (condition-case err
+                      (progn (cj/term-tmux-detach) nil)
+                    (error err))))))
+
+(ert-deftest test-eat-config-tmux-detach-bound-on-term-map ()
+  "Normal: the command sits on the terminal map next to copy-mode (\"c\")."
+  (should (eq (keymap-lookup cj/term-map "d") #'cj/term-tmux-detach)))
+
+(provide 'test-term-tmux-detach)
+;;; test-term-tmux-detach.el ends here