Invisible widgets now dont render

examples/dev_sandbox.py

@@ -0,0 +1,32 @@
+from rich.console import RenderableType
+from rich.panel import Panel
+
+from textual.app import App
+from textual.widget import Widget
+
+
+class PanelWidget(Widget):
+    def render(self) -> RenderableType:
+        return Panel("hello world!", title="Title")
+
+
+class BasicApp(App):
+    """Sandbox application used for testing/development by Textual developers"""
+
+    def on_load(self):
+        """Bind keys here."""
+        self.bind("tab", "toggle_class('#sidebar', '-active')")
+        self.bind("a", "toggle_class('#header', '-visible')")
+        self.bind("c", "toggle_class('#content', '-content-visible')")
+
+    def on_mount(self):
+        """Build layout here."""
+        self.mount(
+            header=Widget(),
+            content=PanelWidget(),
+            footer=Widget(),
+            sidebar=Widget(),
+        )
+
+
+BasicApp.run(css_file="test_app.css", watch_css=True, log="textual.log")