fix(ui-navigation): clear dedicated before toggling window split

toggle-window-split swapped buffers between two windows via two set-window-buffer calls. If either window was strongly dedicated (e.g. *Org Agenda* via the display-buffer-alist rule), the dedicated window rejected the second swap. Both panes ended up showing the dedicated buffer. The non-dedicated buffer never made it across.

The fix clears dedicated on both windows before the swap. The user explicitly invoked a layout change, so preserving per-window dedicated through the operation would just re-trigger the same wedge on the next toggle.

Tests in tests/test-ui-navigation--toggle-window-split.el cover the no-dedicated baseline, the bug-trigger (dedicated selected window), the post-toggle cleared state, and the one-window / three-window no-op boundaries.

1 files changed, 110 insertions, 0 deletions

diff --git a/tests/test-ui-navigation--toggle-window-split.el b/tests/test-ui-navigation--toggle-window-split.el
new file mode 100644
index 00000000..d93f7aad
--- /dev/null
+++ b/tests/test-ui-navigation--toggle-window-split.el
@@ -0,0 +1,110 @@
+;;; test-ui-navigation--toggle-window-split.el --- Tests for toggle-window-split -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for `toggle-window-split' in ui-navigation.el.
+;;
+;; The function flips a 2-window split between horizontal and vertical
+;; orientation.  Historically it failed when one of the two windows was
+;; marked strongly-dedicated (e.g. *Org Agenda*) -- `set-window-buffer'
+;; rejects buffer swaps on strongly-dedicated windows, so the
+;; non-dedicated buffer never made it across and both windows ended up
+;; showing the dedicated one.  The fix clears dedicated on both windows
+;; before the swap.
+
+;;; Code:
+
+(require 'ert)
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'ui-navigation)
+
+(defmacro test-toggle--with-buffers (&rest body)
+  "Bind `buf-a' and `buf-b' to fresh buffers around BODY, killing both on exit.
+The buffers are visible to BODY by name so it can pass them into the
+window-setup macro without re-binding."
+  (declare (indent 0))
+  `(let ((buf-a (generate-new-buffer "*test-toggle-a*"))
+         (buf-b (generate-new-buffer "*test-toggle-b*")))
+     (unwind-protect
+         (progn ,@body)
+       (kill-buffer buf-a)
+       (kill-buffer buf-b))))
+
+(defmacro test-toggle--with-two-windows (buf-a buf-b split-fn &rest body)
+  "Set up two windows displaying BUF-A and BUF-B via SPLIT-FN, run BODY.
+BUF-A goes in the top/left window, BUF-B in the bottom/right.  Point
+ends in the BUF-B window -- the 2nd / lower-or-right one -- because
+that's the position that triggers the dedicated-window bug.  The
+original repro is F8-then-toggle, which leaves selection in the agenda
+(below the source buffer) when the toggle runs.  Restores the outer
+window config when BODY returns.  SPLIT-FN is `split-window-below' or
+`split-window-right'."
+  (declare (indent 3))
+  `(save-window-excursion
+     (delete-other-windows)
+     (set-window-buffer (selected-window) ,buf-a)
+     (funcall ,split-fn)
+     (other-window 1)
+     (set-window-buffer (selected-window) ,buf-b)
+     ,@body))
+
+;;; Normal Cases
+
+(ert-deftest test-ui-navigation-toggle-split-normal-no-dedicated-keeps-both-buffers ()
+  "Normal: vertical split, no dedicated -- toggle preserves both buffers."
+  (test-toggle--with-buffers
+    (test-toggle--with-two-windows buf-a buf-b #'split-window-below
+      (toggle-window-split)
+      (let ((bufs (mapcar #'window-buffer (window-list))))
+        (should (memq buf-a bufs))
+        (should (memq buf-b bufs))))))
+
+(ert-deftest test-ui-navigation-toggle-split-normal-dedicated-selected-window-keeps-both-buffers ()
+  "Normal: selected window dedicated -- toggle still distributes both buffers.
+This is the bug scenario from the *Org Agenda* report: when the
+selected window is strongly-dedicated, the internal
+`set-window-buffer' for the non-matching buffer is rejected and both
+windows end up showing the dedicated buffer.  After the fix, dedicated
+is cleared first and both buffers reach their target windows."
+  (test-toggle--with-buffers
+    (test-toggle--with-two-windows buf-a buf-b #'split-window-below
+      (set-window-dedicated-p (selected-window) t)
+      (toggle-window-split)
+      (let ((bufs (mapcar #'window-buffer (window-list))))
+        (should (memq buf-a bufs))
+        (should (memq buf-b bufs))))))
+
+(ert-deftest test-ui-navigation-toggle-split-normal-clears-dedicated-state ()
+  "Normal: dedicated state is cleared after toggle.
+The user invoked a layout change; preserving per-window dedicated
+through the swap would re-introduce the same wedge on the next toggle."
+  (test-toggle--with-buffers
+    (test-toggle--with-two-windows buf-a buf-b #'split-window-below
+      (set-window-dedicated-p (selected-window) t)
+      (toggle-window-split)
+      (dolist (w (window-list))
+        (should-not (window-dedicated-p w))))))
+
+;;; Boundary Cases
+
+(ert-deftest test-ui-navigation-toggle-split-boundary-single-window-noop ()
+  "Boundary: a single window in the frame -- toggle is a no-op."
+  (save-window-excursion
+    (delete-other-windows)
+    (toggle-window-split)
+    (should (= 1 (length (window-list))))))
+
+(ert-deftest test-ui-navigation-toggle-split-boundary-three-windows-noop ()
+  "Boundary: three windows in the frame -- toggle is a no-op."
+  (test-toggle--with-buffers
+    (save-window-excursion
+      (delete-other-windows)
+      (set-window-buffer (selected-window) buf-a)
+      (split-window-below)
+      (split-window-right)
+      (let ((before-count (length (window-list))))
+        (toggle-window-split)
+        (should (= before-count (length (window-list))))))))
+
+(provide 'test-ui-navigation--toggle-window-split)
+;;; test-ui-navigation--toggle-window-split.el ends here