feat(slack): open rooms in another window, never the selected one

slack-buffer-function defaulted to switch-to-buffer-other-window, which gives no real guarantee about placement: with three or more panes it picks a least-recently-used window, and it offers nothing that keeps Slack out of the window point is in. So opening a room in a split could land it wherever, including over the buffer I was working in.

I set slack-buffer-function to cj/slack--display-buffer, a pop-to-buffer call with inhibit-same-window and a reuse / use-some-window / pop-up-window action list. In a split it reuses one of the other windows and leaves the selected window alone; with a lone window it splits. Tests cover both the split-placement case and the selected-window-preserved invariant.

2 files changed, 65 insertions, 0 deletions

diff --git a/modules/slack-config.el b/modules/slack-config.el
index 48af9c93..0902ef35 100644
--- a/modules/slack-config.el
+++ b/modules/slack-config.el
@@ -102,6 +102,20 @@
   (slack-ws-close)
   (message "Slack disconnected."))
 
+(defun cj/slack--display-buffer (buffer)
+  "Display Slack BUFFER in another window, never the selected one.
+With a split, reuse one of the other windows rather than taking over the
+window point is in or always spawning a fresh split; with a lone window,
+split.  Wired as `slack-buffer-function' so opening a room or thread lands
+beside the current work instead of clobbering it.  The default
+`switch-to-buffer-other-window' picks a least-recently-used window with three
+or more panes; this pins the choice to any non-selected window."
+  (pop-to-buffer buffer
+                 '((display-buffer-reuse-window
+                    display-buffer-use-some-window
+                    display-buffer-pop-up-window)
+                   (inhibit-same-window . t))))
+
 (use-package slack
   :defer t
   :commands (slack-start slack-select-rooms slack-select-unread-rooms
@@ -114,6 +128,9 @@
   ;; Re-enable if emojify/circe fix the interaction. (2026-03-16)
   (slack-buffer-emojify nil)
   (slack-prefer-current-team t)
+  ;; Open rooms/threads in another window, never the selected one (see
+  ;; cj/slack--display-buffer) so Slack lands beside the current work in a split.
+  (slack-buffer-function #'cj/slack--display-buffer)
   :config
   (setq slack-message-custom-notifier #'cj/slack-notify))
 
diff --git a/tests/test-slack-config-display.el b/tests/test-slack-config-display.el
new file mode 100644
index 00000000..24128ddc
--- /dev/null
+++ b/tests/test-slack-config-display.el
@@ -0,0 +1,48 @@
+;;; test-slack-config-display.el --- Slack buffer placement -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; cj/slack--display-buffer is wired as `slack-buffer-function' so opening a
+;; Slack room or thread shows it in another window and never replaces the
+;; selected window's buffer.  These exercise the placement directly with plain
+;; buffers (no live Slack), using a side-by-side split so the window math is
+;; reliable under `emacs --batch'.
+
+;;; Code:
+
+(require 'ert)
+(require 'slack-config)
+
+(ert-deftest test-slack-config-display-buffer-uses-other-window-when-split ()
+  "Normal: with a split, the buffer lands in a non-selected window."
+  (let ((a (get-buffer-create "*slack-disp-a*"))
+        (b (get-buffer-create "*slack-disp-b*")))
+    (unwind-protect
+        (save-window-excursion
+          (delete-other-windows)
+          (set-window-buffer (selected-window) a)
+          (let ((win-a (selected-window)))
+            (split-window win-a nil t)   ;; side-by-side, batch-friendly
+            (cj/slack--display-buffer b)
+            (let ((win-b (get-buffer-window b)))
+              (should win-b)
+              (should-not (eq win-b win-a)))))
+      (ignore-errors (kill-buffer a))
+      (ignore-errors (kill-buffer b)))))
+
+(ert-deftest test-slack-config-display-buffer-keeps-selected-window ()
+  "Boundary: displaying does not replace the selected window's buffer."
+  (let ((a (get-buffer-create "*slack-disp-a*"))
+        (b (get-buffer-create "*slack-disp-b*")))
+    (unwind-protect
+        (save-window-excursion
+          (delete-other-windows)
+          (set-window-buffer (selected-window) a)
+          (let ((win-a (selected-window)))
+            (split-window win-a nil t)
+            (cj/slack--display-buffer b)
+            (should (eq (window-buffer win-a) a))))
+      (ignore-errors (kill-buffer a))
+      (ignore-errors (kill-buffer b)))))
+
+(provide 'test-slack-config-display)
+;;; test-slack-config-display.el ends here