Add a test for inheriting from a ListView

Added to test the fail in Textualize#1588. Any fix for this should cause this test to pass.

tests/listview/test_inherit_listview.py

@@ -0,0 +1,26 @@
+from textual.app import App, ComposeResult
+from textual.widgets import ListView, ListItem, Label
+
+
+class MyListView(ListView):
+    def compose(self) -> ComposeResult:
+        """Compose the child widgets."""
+        for n in range(20):
+            yield ListView(Label(f"This is item {n}"))
+
+
+class ListViewApp(App[None]):
+    """ListView test app."""
+
+    def compose(self) -> ComposeResult:
+        """Compose the child widgets."""
+        yield MyListView()
+
+
+async def test_inherited_list_view() -> None:
+    """A self-populating inherited ListView should work as normal."""
+    async with ListViewApp().run_test() as pilot:
+        await pilot.press("tab")
+        assert pilot.app.query_one(MyListView).index == 0
+        await pilot.press("down")
+        assert pilot.app.query_one(MyListView).index == 1