Fix a minor todo-mode regression

* lisp/calendar/todo-mode.el (todo-get-overlay): Wrap in
save-excursion.  This fixes a regression introduced by the fix
for bug#27609, whereby trying to raise the priority of the
first item or lower the priority of the last item, which
should be noops, moves point to the item's start.  Clarify
comment.

* test/lisp/calendar/todo-mode-tests.el
(todo-test-raise-lower-priority): Add test cases for trying to
raise first item and lower last item.
(with-todo-test): Clear abbreviated-home-dir, since we change HOME.
(todo-test-toggle-item-header02): Remove ":expected-result
:failed" and tests of point after todo-next-item, since the
effect when using Todo mode is not reproducible in the test
environment.  Add commentary about this.

1 files changed, 15 insertions, 14 deletions

diff --git a/lisp/calendar/todo-mode.el b/lisp/calendar/todo-mode.el
index e39fee5bfa1..ba7389c07a2 100644
--- a/lisp/calendar/todo-mode.el
+++ b/lisp/calendar/todo-mode.el
@@ -5381,20 +5381,21 @@ marked) not done todo items."
 
 (defun todo-get-overlay (val)
   "Return the overlay at point whose `todo' property has value VAL."
-  ;; When headers are hidden, the display engine makes item's start
-  ;; inaccessible to commands, so go there here, if necessary, in
-  ;; order to check for prefix and header overlays.
-  (when (memq val '(prefix header))
-    (unless (looking-at todo-item-start) (todo-item-start)))
-  ;; Use overlays-in to find prefix overlays and check over two
-  ;; positions to find done separator overlay.
-  (let ((ovs (overlays-in (point) (1+ (point))))
-        ov)
-    (catch 'done
-      (while ovs
-        (setq ov (pop ovs))
-        (when (eq (overlay-get ov 'todo) val)
-          (throw 'done ov))))))
+  (save-excursion
+    ;; When headers are hidden, the display engine makes item's start
+    ;; inaccessible to commands, so then we have to go there
+    ;; non-interactively to check for prefix and header overlays.
+    (when (memq val '(prefix header))
+      (unless (looking-at todo-item-start) (todo-item-start)))
+    ;; Use overlays-in to find prefix overlays and check over two
+    ;; positions to find done separator overlay.
+    (let ((ovs (overlays-in (point) (1+ (point))))
+          ov)
+      (catch 'done
+        (while ovs
+          (setq ov (pop ovs))
+          (when (eq (overlay-get ov 'todo) val)
+            (throw 'done ov)))))))
 
 (defun todo-marked-item-p ()
   "Non-nil if this item begins with `todo-item-mark'.