Fix map invalidation (#2129)

* Invalidation logic

* comment and docstring

* changelog

* snapshot

tests/snapshot_tests/snapshot_apps/tabs_invalidate.py

@@ -0,0 +1,24 @@
+from textual.app import App, ComposeResult
+from textual.widgets import Label, TabbedContent, TabPane
+
+
+class TabApp(App):
+    CSS = """
+    TabPane {
+    border: solid blue;
+    }
+    """
+
+    def compose(self) -> ComposeResult:
+        with TabbedContent():
+            with TabPane("Tab 1", id="tab-1"):
+                yield Label("hello")
+                yield Label("hello")
+                yield Label("hello")
+            with TabPane("Tab 2", id="tab-2"):
+                yield Label("world")
+
+
+if __name__ == "__main__":
+    app = TabApp()
+    app.run()