fixed some rendering glitches

examples/tree.py

@@ -0,0 +1,32 @@
+import json
+
+from textual.app import App, ComposeResult
+from textual.widgets import Header, Footer, Tree
+
+
+with open("food.json") as data_file:
+    data = json.load(data_file)
+
+from rich import print
+
+print(data)
+
+
+class TreeApp(App):
+
+    BINDINGS = [("a", "add", "Add node")]
+
+    def compose(self) -> ComposeResult:
+        yield Header()
+        yield Footer()
+        yield Tree("Root")
+
+    def action_add(self) -> None:
+        tree = self.query_one(Tree)
+
+        tree.add_json(data)
+
+
+if __name__ == "__main__":
+    app = TreeApp()
+    app.run()