enforce mounting

CHANGELOG.md

@@ -11,6 +11,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - Fixed `Footer` grid size https://github.com/Textualize/textual/pull/4545
 
+### Changed
+
+- Attempting to mount on a non-mounted widget now raises a MountError https://github.com/Textualize/textual/pull/4547
+
 ## [0.63.2] - 2024-05-23
 
 ### Fixed

src/textual/app.py

@@ -769,6 +769,11 @@ async def stop_animation(self, attribute: str, complete: bool = True) -> None:
         """
         await self._animator.stop_animation(self, attribute, complete)
 
+    @property
+    def is_dom_root(self) -> bool:
+        """Is this a root node (i.e. the App)?"""
+        return True
+
     @property
     def debug(self) -> bool:
         """Is debug mode enabled?"""

src/textual/message_pump.py

@@ -215,6 +215,11 @@ def message_queue_size(self) -> int:
         """The current size of the message queue."""
         return self._message_queue.qsize()
 
+    @property
+    def is_dom_root(self):
+        """Is this a root node (i.e. the App)?"""
+        return False
+
     @property
     def app(self) -> "App[object]":
         """
@@ -239,6 +244,16 @@ def app(self) -> "App[object]":
             active_app.set(node)
             return node
 
+    @property
+    def _is_linked_to_app(self) -> bool:
+        """Is this node linked to the app through the DOM?"""
+        node: MessagePump | None = self
+
+        while (node := node._parent) is not None:
+            if node.is_dom_root:
+                return True
+        return False
+
     @property
     def is_parent_active(self) -> bool:
         """Is the parent active?"""

src/textual/widget.py

@@ -934,6 +934,8 @@ def mount(
             Only one of ``before`` or ``after`` can be provided. If both are
             provided a ``MountError`` will be raised.
         """
+        if not self._is_linked_to_app:
+            raise MountError(f"Can't mount widget(s) before {self!r} is mounted")
         # Check for duplicate IDs in the incoming widgets
         ids_to_mount = [widget.id for widget in widgets if widget.id is not None]
         unique_ids = set(ids_to_mount)
@@ -1124,7 +1126,8 @@ async def recompose(self) -> None:
         if self._parent is not None:
             async with self.batch():
                 await self.query("*").exclude(".-textual-system").remove()
-                await self.mount_all(compose(self))
+                if self._is_linked_to_app:
+                    await self.mount_all(compose(self))
 
     def _post_register(self, app: App) -> None:
         """Called when the instance is registered.

tests/test_widget_mounting.py

@@ -114,3 +114,10 @@ async def test_mount_via_app() -> None:
         await pilot.app.mount_all(widgets)
         with pytest.raises(TooManyMatches):
             await pilot.app.mount(Static(), before="Static")
+
+
+def test_mount_error() -> None:
+    """Mounting a widget on an un-mounted widget should raise an error."""
+    with pytest.raises(MountError):
+        widget = Widget()
+        widget.mount(Static())