test for is_attached

src/textual/message_pump.py

@@ -125,8 +125,17 @@ class MessagePump(metaclass=MessagePumpMeta):
 
     @property
     def is_attached(self) -> bool:
-        """Check the node is attached to the DOM"""
-        return self._parent is not None
+        """Check the node is attached to the app via the DOM."""
+
+        from .app import App
+
+        node = self
+
+        while not isinstance(node, App):
+            if node._parent is None:
+                return False
+            node = node._parent
+        return True
 
     def _attach(self, parent: MessagePump) -> None:
         """Set the parent, and therefore attach this node to the tree.

tests/test_widget_removing.py

@@ -8,9 +8,13 @@ from textual.containers import Container
 async def test_remove_single_widget():
     """It should be possible to the only widget on a screen."""
     async with App().run_test() as pilot:
-        await pilot.app.mount(Static())
+        widget = Static()
+        assert not widget.is_attached
+        await pilot.app.mount(widget)
+        assert widget.is_attached
         assert len(pilot.app.screen.children) == 1
         await pilot.app.query_one(Static).remove()
+        assert not widget.is_attached
         assert len(pilot.app.screen.children) == 0