Add a unit test for testing Textualize#2363

tests/directory_tree/test_early_show_root.py

@@ -0,0 +1,16 @@
+from textual.app import App, ComposeResult
+from textual.widgets import DirectoryTree
+
+
+class DirectoryTreeApp(App[None]):
+    def compose(self) -> ComposeResult:
+        tree = DirectoryTree(".")
+        tree.show_root = True
+        yield tree
+
+
+async def test_managed_to_set_show_root_before_mounted() -> None:
+    """https://github.com/Textualize/textual/issues/2363"""
+    async with DirectoryTreeApp().run_test() as pilot:
+        assert isinstance(pilot.app.query_one(DirectoryTree), DirectoryTree)
+        assert pilot.app.query_one(DirectoryTree).show_root is True