feat(ui-navigation): split from the dashboard opens scratch, not the dashboard

C-x 2 / C-x 3 already show the dashboard in the new window while point stays put. The one dead spot was splitting from the dashboard itself, which put the dashboard in both panes. Now the new window shows *scratch* when the current buffer is the dashboard, and the dashboard everywhere else. I pulled the choice into a pure predicate (cj/--split-from-dashboard-p) and a companion helper, both tested.

2 files changed, 40 insertions, 4 deletions

diff --git a/modules/ui-navigation.el b/modules/ui-navigation.el
index e9c9eaf26..d8d7162e2 100644
--- a/modules/ui-navigation.el
+++ b/modules/ui-navigation.el
@@ -118,15 +118,30 @@ window.  Return the new window."
       (set-window-buffer new buffer))
     new))
 
+(defun cj/--split-from-dashboard-p (buffer-name)
+  "Return non-nil when BUFFER-NAME is the dashboard.
+Splitting from the dashboard shows *scratch* in the new window instead of
+the dashboard again."
+  (equal buffer-name "*dashboard*"))
+
+(defun cj/--split-companion-buffer ()
+  "Buffer to show in the new window after a C-x 2 / C-x 3 split.
+The dashboard, or the *scratch* buffer when splitting from the dashboard."
+  (if (cj/--split-from-dashboard-p (buffer-name))
+      (get-scratch-buffer-create)
+    (cj/--dashboard-buffer)))
+
 (defun cj/split-below-with-dashboard ()
-  "Split below and show the dashboard in the new window; stay in this one."
+  "Split below and show the companion buffer in the new window; stay in this one.
+The companion is the dashboard, or *scratch* when splitting from the dashboard."
   (interactive)
-  (cj/--split-show-buffer #'split-window-below (cj/--dashboard-buffer)))
+  (cj/--split-show-buffer #'split-window-below (cj/--split-companion-buffer)))
 
 (defun cj/split-right-with-dashboard ()
-  "Split right and show the dashboard in the new window; stay in this one."
+  "Split right and show the companion buffer in the new window; stay in this one.
+The companion is the dashboard, or *scratch* when splitting from the dashboard."
   (interactive)
-  (cj/--split-show-buffer #'split-window-right (cj/--dashboard-buffer)))
+  (cj/--split-show-buffer #'split-window-right (cj/--split-companion-buffer)))
 
 (keymap-global-set "C-x 2" #'cj/split-below-with-dashboard)
 (keymap-global-set "C-x 3" #'cj/split-right-with-dashboard)
diff --git a/tests/test-ui-navigation--split-dashboard.el b/tests/test-ui-navigation--split-dashboard.el
index b815a4c59..407335f80 100644
--- a/tests/test-ui-navigation--split-dashboard.el
+++ b/tests/test-ui-navigation--split-dashboard.el
@@ -54,6 +54,27 @@
     (should (eq (car captured) #'split-window-right))
     (should (eq (cadr captured) 'dashboard))))
 
+(ert-deftest test-ui-navigation-split-from-dashboard-p ()
+  "Normal/Boundary: only the dashboard buffer routes the companion to *scratch*."
+  (should (cj/--split-from-dashboard-p "*dashboard*"))
+  (should-not (cj/--split-from-dashboard-p "todo.org"))
+  (should-not (cj/--split-from-dashboard-p "*scratch*")))
+
+(ert-deftest test-ui-navigation-split-companion-scratch-from-dashboard ()
+  "Normal: splitting from the dashboard yields the *scratch* buffer, not the
+dashboard again."
+  (cl-letf (((symbol-function 'cj/--split-from-dashboard-p) (lambda (_) t))
+            ((symbol-function 'get-scratch-buffer-create) (lambda () 'scratch))
+            ((symbol-function 'cj/--dashboard-buffer) (lambda () 'dashboard)))
+    (should (eq (cj/--split-companion-buffer) 'scratch))))
+
+(ert-deftest test-ui-navigation-split-companion-dashboard-otherwise ()
+  "Normal: splitting from any other buffer yields the dashboard."
+  (cl-letf (((symbol-function 'cj/--split-from-dashboard-p) (lambda (_) nil))
+            ((symbol-function 'get-scratch-buffer-create) (lambda () 'scratch))
+            ((symbol-function 'cj/--dashboard-buffer) (lambda () 'dashboard)))
+    (should (eq (cj/--split-companion-buffer) 'dashboard))))
+
 (ert-deftest test-ui-navigation-dashboard-buffer-returns-existing ()
   "Boundary: cj/--dashboard-buffer returns an existing *dashboard* without opening."
   (let ((db (get-buffer-create "*dashboard*"))