diff --git a/src/textual/app.py b/src/textual/app.py
index c671cb216..d6a0d766d 100644
--- a/src/textual/app.py
+++ b/src/textual/app.py
@@ -414,10 +414,6 @@ class App(Generic[ReturnType], DOMNode):
         """ReturnType | None: The return type of the app."""
         return self._return_value
 
-    def _post_mount(self):
-        """Called after the object has been mounted."""
-        Reactive._initialize_object(self)
-
     def animate(
         self,
         attribute: str,
diff --git a/src/textual/dom.py b/src/textual/dom.py
index b6129daed..c56c45b34 100644
--- a/src/textual/dom.py
+++ b/src/textual/dom.py
@@ -210,6 +210,10 @@ class DOMNode(MessagePump):
         styles = self._component_styles[name]
         return styles
 
+    def _post_mount(self):
+        """Called after the object has been mounted."""
+        Reactive._initialize_object(self)
+
     @property
     def _node_bases(self) -> Iterator[Type[DOMNode]]:
         """Iterator[Type[DOMNode]]: The DOMNode bases classes (including self.__class__)"""
diff --git a/src/textual/reactive.py b/src/textual/reactive.py
index 3d3e79de6..c2863deab 100644
--- a/src/textual/reactive.py
+++ b/src/textual/reactive.py
@@ -11,7 +11,6 @@ from typing import (
     Generic,
     Type,
     TypeVar,
-    Union,
 )
 
 import rich.repr
@@ -21,10 +20,9 @@ from ._callback import count_parameters
 from ._types import MessageTarget
 
 if TYPE_CHECKING:
-    from .app import App
-    from .widget import Widget
+    from .dom import DOMNode
 
-    Reactable = Union[Widget, App]
+    Reactable = DOMNode
 
 ReactiveType = TypeVar("ReactiveType")
 
diff --git a/src/textual/widget.py b/src/textual/widget.py
index df297fcd8..5bd204c9e 100644
--- a/src/textual/widget.py
+++ b/src/textual/widget.py
@@ -360,10 +360,6 @@ class Widget(DOMNode):
     def offset(self, offset: Offset) -> None:
         self.styles.offset = ScalarOffset.from_offset(offset)
 
-    def _post_mount(self):
-        """Called after the object has been mounted."""
-        Reactive._initialize_object(self)
-
     ExpectType = TypeVar("ExpectType", bound="Widget")
 
     @overload