diff --git a/src/textual/_compositor.py b/src/textual/_compositor.py
index 7ca7f18432..636db18581 100644
--- a/src/textual/_compositor.py
+++ b/src/textual/_compositor.py
@@ -486,9 +486,8 @@ def full_map(self) -> CompositorMap:
             return {}
         if self._full_map_invalidated:
             self._full_map_invalidated = False
-            old_widgets = self._full_map.keys()
-            map, widgets = self._arrange_root(self.root, self.size, visible_only=False)
-            self._new_widgets.update(map.keys() - old_widgets)
+            map, _widgets = self._arrange_root(self.root, self.size, visible_only=False)
+            self._new_widgets.update(map.keys() - self._full_map.keys())
             self._full_map = map
             self._visible_widgets = None
             self._visible_map = None