header fixes, and lazy queries

sandbox/will/footer.py

@@ -1,13 +1,17 @@
 from textual.app import App
-from textual.widgets import Footer
+from textual.widgets import Header, Footer
 
 
 class FooterApp(App):
     def on_mount(self):
-        self.dark = True
+        self.sub_title = "Header and footer example"
         self.bind("b", "app.bell", description="Play the Bell")
+        self.bind("d", "dark", description="Toggle dark")
         self.bind("f1", "app.bell", description="Hello World")
-        self.bind("f2", "app.bell", description="Do something")
+
+    def action_dark(self):
+        self.dark = not self.dark
 
     def compose(self):
+        yield Header()
         yield Footer()