Ensure ListView.index is correct in all post-mount situations

This seeks to fix Textualize#1588 by ensuring that the index property is set to an
acceptable value after any items have been mounted within the ListView, thus
ensuring that an inherited ListView, which uses compose to add items rather
than having them passed via __init__, works as expected.

src/textual/widgets/_list_view.py

@@ -52,6 +52,15 @@ def __init__(
         super().__init__(*children, name=name, id=id, classes=classes)
         self.index = initial_index
 
+    def on_mount(self) -> None:
+        """Ensure the ListView is fully-settled after mounting."""
+        # If someone inherits from ListView, they might be populating it
+        # from Widget.compose rather than from passing the ListItems to the
+        # initialisation call. Given that watch_index does all the work,
+        # let's just tickle it again after the DOM has been spun up and all
+        # children of this widget will have been mounted.
+        self.index = self.index or 0
+
     @property
     def highlighted_child(self) -> ListItem | None:
         """ListItem | None: The currently highlighted ListItem,