Moving child before/after self is a no-op.

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Changed
 
 - App `title` and `sub_title` attributes can be set to any type https://github.com/Textualize/textual/issues/2521
+- Using `Widget.move_child` where the target and the child being moved are the same is now a no-op https://github.com/Textualize/textual/issues/1743
 
 ### Fixed
 

src/textual/widget.py

@@ -795,19 +795,15 @@ def move_child(
 
         Args:
             child: The child widget to move.
-            before: Optional location to move before. An `int` is the index
-                of the child to move before, a `str` is a `query_one` query to
-                find the widget to move before.
-            after: Optional location to move after. An `int` is the index
-                of the child to move after, a `str` is a `query_one` query to
-                find the widget to move after.
+            before: Child widget or location index to move before.
+            after: Child widget or location index to move after.
 
         Raises:
             WidgetError: If there is a problem with the child or target.
 
         Note:
-            Only one of ``before`` or ``after`` can be provided. If neither
-            or both are provided a ``WidgetError`` will be raised.
+            Only one of `before` or `after` can be provided. If neither
+            or both are provided a `WidgetError` will be raised.
         """
 
         # One or the other of before or after are required. Can't do
@@ -817,6 +813,10 @@ def move_child(
         elif before is not None and after is not None:
             raise WidgetError("Only one of `before` or `after` can be handled.")
 
+        # We short-circuit the no-op, otherwise it will error later down the road.
+        if child is before or child is after:
+            return
+
         def _to_widget(child: int | Widget, called: str) -> Widget:
             """Ensure a given child reference is a Widget."""
             if isinstance(child, int):

tests/test_widget_child_moving.py

@@ -42,22 +42,18 @@ async def test_move_child_to_outside() -> None:
             pilot.app.screen.move_child(child, before=Widget())
 
 
-@pytest.mark.xfail(
-    strict=True, reason="https://github.com/Textualize/textual/issues/1743"
-)
 async def test_move_child_before_itself() -> None:
     """Test moving a widget before itself."""
+
     async with App().run_test() as pilot:
         child = Widget(Widget())
         await pilot.app.mount(child)
         pilot.app.screen.move_child(child, before=child)
 
 
-@pytest.mark.xfail(
-    strict=True, reason="https://github.com/Textualize/textual/issues/1743"
-)
 async def test_move_child_after_itself() -> None:
     """Test moving a widget after itself."""
+    # Regression test for https://github.com/Textualize/textual/issues/1743
     async with App().run_test() as pilot:
         child = Widget(Widget())
         await pilot.app.mount(child)