From 3acdb28ed69ce27aa3cd4086cfe9714fc1074431 Mon Sep 17 00:00:00 2001
From: Craig Jennings <c@cjennings.net>
Date: Tue, 26 May 2026 14:28:18 -0500
Subject: feat(mail): keep mu4e's main view from deleting the window split

mu4e's main view displays with a display-buffer-full-frame action, which tears down every other window on launch, so opening mu4e from a split collapsed the layout. mu4e's own mu4e-display-buffer docstring points to display-buffer-alist as the supported override.

I added an entry routing the *mu4e-main* buffer to the current window (reuse a window already showing it first, then same-window), so launching mu4e in a split keeps the rest of the layout intact. It's registered eagerly rather than inside mu4e's deferred config so it applies on the first launch. Tests cover the entry registration and that the main buffer no longer collapses a split under mu4e's full-frame action.
---
 modules/mail-config.el                | 12 ++++++++++
 tests/test-mail-config-mu4e-window.el | 42 +++++++++++++++++++++++++++++++++++
 2 files changed, 54 insertions(+)
 create mode 100644 tests/test-mail-config-mu4e-window.el

diff --git a/modules/mail-config.el b/modules/mail-config.el
index a498844e..f71d6eeb 100644
--- a/modules/mail-config.el
+++ b/modules/mail-config.el
@@ -124,6 +124,18 @@ Prompts user for the action when executing."
 (autoload 'mu4e "mu4e" "Launch mu4e email client." t)
 (keymap-global-set "C-c m" #'mu4e)
 
+;; mu4e's main view uses a display-buffer-full-frame action (see
+;; mu4e-display-buffer), which deletes the window split on launch.  Per that
+;; function's own docstring, the supported way to change it is
+;; display-buffer-alist.  Route the main buffer to the current window instead
+;; (reuse a window already showing it first), so launching mu4e in a split
+;; leaves the rest of the layout intact.  Registered eagerly rather than in
+;; mu4e's deferred :config so it applies on the first launch.
+(add-to-list 'display-buffer-alist
+             '("\\`\\*mu4e-main\\*\\'"
+               (display-buffer-reuse-window display-buffer-same-window)
+               (inhibit-same-window . nil)))
+
 (use-package mu4e
   :ensure nil  ;; mu4e gets installed by installing 'mu' via the system package manager
   :load-path "/usr/share/emacs/site-lisp/mu4e/"
diff --git a/tests/test-mail-config-mu4e-window.el b/tests/test-mail-config-mu4e-window.el
new file mode 100644
index 00000000..e02a7f9a
--- /dev/null
+++ b/tests/test-mail-config-mu4e-window.el
@@ -0,0 +1,42 @@
+;;; test-mail-config-mu4e-window.el --- mu4e main-view window placement -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; mu4e's main view defaults to a display-buffer-full-frame action that deletes
+;; the window split on launch.  mail-config registers a display-buffer-alist
+;; entry routing the "*mu4e-main*" buffer to the current window instead.  These
+;; verify the entry is registered and that, with it active, displaying the main
+;; buffer under mu4e's own full-frame action no longer collapses a split.
+
+;;; Code:
+
+(require 'ert)
+(require 'mail-config)
+
+(ert-deftest test-mail-config-mu4e-main-entry-registered ()
+  "Normal: the *mu4e-main* alist entry avoids the full-frame action."
+  (let ((entry (assoc "\\`\\*mu4e-main\\*\\'" display-buffer-alist)))
+    (should entry)
+    (should (memq 'display-buffer-same-window (cadr entry)))
+    (should-not (memq 'display-buffer-full-frame (cadr entry)))))
+
+(ert-deftest test-mail-config-mu4e-main-keeps-split ()
+  "Normal: displaying *mu4e-main* under the full-frame action keeps the split.
+The registered alist entry takes precedence over the action passed to
+`display-buffer', so the sibling window survives."
+  (let ((a (get-buffer-create "*mail-win-test-a*"))
+        (main (get-buffer-create "*mu4e-main*")))
+    (unwind-protect
+        (save-window-excursion
+          (delete-other-windows)
+          (set-window-buffer (selected-window) a)
+          (split-window (selected-window) nil t)   ;; side-by-side, batch-friendly
+          (let ((n (count-windows)))
+            (display-buffer main '(display-buffer-reuse-window
+                                   display-buffer-reuse-mode-window
+                                   display-buffer-full-frame))
+            (should (= (count-windows) n))))
+      (ignore-errors (kill-buffer a))
+      (ignore-errors (kill-buffer main)))))
+
+(provide 'test-mail-config-mu4e-window)
+;;; test-mail-config-mu4e-window.el ends here
-- 
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