Invisible widgets now dont render

examples/dev_sandbox.css

@@ -0,0 +1,49 @@
+/* CSS file for dev_sandbox.py */
+
+App > View {
+  docks: side=left/1;
+  text: on #20639b;
+}
+
+Widget:hover {
+  outline: heavy;
+  text: bold !important;
+}
+
+#sidebar {
+  text: #09312e on #3caea3;
+  dock: side;
+  width: 30;
+  offset-x: -100%;
+  transition: offset 500ms in_out_cubic;
+  border-right: outer #09312e;
+}
+
+#sidebar.-active {
+  offset-x: 0;
+}
+
+#header {
+  text: white on #173f5f;
+  height: 3;
+  border: hkey;
+}
+
+#header.-visible {
+  visibility: hidden;
+}
+
+#content {
+  text: white on #20639b;
+  border-bottom: hkey #0f2b41;
+  offset-y: -3;
+}
+
+#content.-content-visible {
+  visibility: hidden;
+}
+
+#footer {
+  text: #3a3009 on #f6d55c;
+  height: 3;
+}

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")