Each class owns its declared method keys, settled once at creation

src/textual/dom.py

@@ -574,8 +574,8 @@ def declared_bindings(cls, mode: type[EditingMode]) -> tuple[Binding, ...]:
     @classmethod
     def method_bindings(cls, mode: type[EditingMode]) -> tuple[Binding, ...]:
         """Keys each class in the MRO declares on its own methods."""
-        return tuple(projected for base in cls.__mro__
-                     for binding in base.__dict__.get("_declared_key_bindings", ())
+        return tuple(projected for base in cls._dom_bases()
+                     for binding in base._declared_key_bindings
                      for projected in binding.for_mode(mode))
 
     @classmethod
@@ -589,19 +589,24 @@ def class_bindings(cls, mode: type[EditingMode]) -> dict[str, list[Binding]]:
         keys: dict[str, list[Binding]] = {}
         for base in cls._binding_bases():
             keys.update(base._own_bindings.project(
-                mode, base.__dict__.get("_declared_key_bindings", ())).key_to_bindings)
+                mode, base._declared_key_bindings).key_to_bindings)
         return keys
 
+    @classmethod
+    def _dom_bases(cls) -> tuple[type[DOMNode], ...]:
+        """The MRO's DOMNode classes, most derived first: object and plain mixins never
+        pass DOMNode.__init_subclass__, so they own no bindings."""
+        return tuple(base for base in cls.__mro__ if issubclass(base, DOMNode))
+
     @classmethod
     def _binding_bases(cls) -> list[type[DOMNode]]:
         """The DOMNode bases whose keys this class inherits, oldest first: the walk restarts
         at any base that does not inherit its ancestors' bindings."""
         bases: list[type[DOMNode]] = []
-        for base in reversed(cls.__mro__):
-            if issubclass(base, DOMNode):
-                if not base._inherit_bindings:
-                    bases.clear()
-                bases.append(base)
+        for base in reversed(cls._dom_bases()):
+            if not base._inherit_bindings:
+                bases.clear()
+            bases.append(base)
         return bases
 
     @classmethod
@@ -1025,6 +1030,8 @@ def __init_subclass__(
         for base in bases:
             css_type_names.add(base.__name__)
         cls._own_bindings = BindingsMap(cls.__dict__.get("BINDINGS", []))
+        # MethodBinding.__set_name__ has already run for this body; a class without one owns none.
+        cls._declared_key_bindings = cls.__dict__.get("_declared_key_bindings", ())
         cls._merged_bindings = cls._merge_bindings()
         cls._css_type_names = frozenset(css_type_names)
         compute_methods = frozenset(