fix(prog-general): open the project todo in the other window when split

C-c p t (cj/open-project-root-todo) called find-file, which always opened todo.org in the selected window, replacing whatever I was looking at. Now it opens in the other window when the frame is split and in the current window when it isn't, through a small cj/--find-file-respecting-split helper. The helper is a top-level defun rather than buried in the projectile :config block so it can be unit-tested without loading projectile.

I left cj/project-switch-actions alone. Opening the todo on a project switch is a different trigger and not what this fixes.

2 files changed, 73 insertions, 1 deletions

diff --git a/modules/prog-general.el b/modules/prog-general.el
index dedf8a80..93a3a95c 100644
--- a/modules/prog-general.el
+++ b/modules/prog-general.el
@@ -137,6 +137,15 @@
 	(unless (file-exists-p projectile-bookmark-file)
 	  (run-at-time "3" nil 'projectile-discover-projects-in-search-path))))
 
+(defun cj/--find-file-respecting-split (file)
+  "Open FILE in another window when the frame is split, else in this window.
+\"Split\" means the selected window is not the only one in its frame, so
+opening the file lands it in the other pane rather than replacing the
+buffer the user is looking at."
+  (if (one-window-p)
+	  (find-file file)
+	(find-file-other-window file)))
+
 (use-package projectile
   :bind-keymap
   ("C-c p" . projectile-command-map)
@@ -171,7 +180,7 @@ If no such file exists there, display a message."
 	(if-let ((root (projectile-project-root)))
 		(let ((file (cj/find-project-root-file "^todo\\.org$")))
 		  (if file
-			  (find-file (expand-file-name file root))
+			  (cj/--find-file-respecting-split (expand-file-name file root))
 			(message "No todo.org in project root: %s" root)))
 	  (message "Not in a Projectile project")))
 
diff --git a/tests/test-prog-general--find-file-respecting-split.el b/tests/test-prog-general--find-file-respecting-split.el
new file mode 100644
index 00000000..6d45c51c
--- /dev/null
+++ b/tests/test-prog-general--find-file-respecting-split.el
@@ -0,0 +1,63 @@
+;;; test-prog-general--find-file-respecting-split.el --- split-aware file open -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; `cj/--find-file-respecting-split' opens a file in another window when the
+;; frame is split and in the current window otherwise.  It backs
+;; `cj/open-project-root-todo' (C-c p t) so the project todo lands in the
+;; other pane instead of replacing the buffer in the selected window.
+;;
+;; `find-file' / `find-file-other-window' are stubbed -- they are the file-I/O
+;; boundary -- while the window layout is real.
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'prog-general)
+
+(ert-deftest test-prog-general--find-file-respecting-split-single-window-uses-current ()
+  "Normal: a single-window frame opens the file in the current window."
+  (save-window-excursion
+    (delete-other-windows)
+    (let (current-arg other-called)
+      (cl-letf (((symbol-function 'find-file)
+                 (lambda (f) (setq current-arg f)))
+                ((symbol-function 'find-file-other-window)
+                 (lambda (_f) (setq other-called t))))
+        (cj/--find-file-respecting-split "/tmp/proj/todo.org"))
+      (should (equal current-arg "/tmp/proj/todo.org"))
+      (should-not other-called))))
+
+(ert-deftest test-prog-general--find-file-respecting-split-two-windows-uses-other ()
+  "Normal: a two-window split opens the file in the other window."
+  (save-window-excursion
+    (delete-other-windows)
+    (split-window-right)
+    (let (other-arg current-called)
+      (cl-letf (((symbol-function 'find-file-other-window)
+                 (lambda (f) (setq other-arg f)))
+                ((symbol-function 'find-file)
+                 (lambda (_f) (setq current-called t))))
+        (cj/--find-file-respecting-split "/tmp/proj/todo.org"))
+      (should (equal other-arg "/tmp/proj/todo.org"))
+      (should-not current-called))))
+
+(ert-deftest test-prog-general--find-file-respecting-split-three-windows-uses-other ()
+  "Boundary: more than two windows still counts as split -> other window."
+  (save-window-excursion
+    (delete-other-windows)
+    (split-window-right)
+    (split-window-below)
+    (let (other-called current-called)
+      (cl-letf (((symbol-function 'find-file-other-window)
+                 (lambda (_f) (setq other-called t)))
+                ((symbol-function 'find-file)
+                 (lambda (_f) (setq current-called t))))
+        (cj/--find-file-respecting-split "/tmp/proj/todo.org"))
+      (should other-called)
+      (should-not current-called))))
+
+(provide 'test-prog-general--find-file-respecting-split)
+;;; test-prog-general--find-file-respecting-split.el ends here