Build a node's CSS ancestry only when a selector needs it

src/textual/css/stylesheet.py

@@ -637,7 +637,6 @@ def apply(
 
             cache_key: tuple | None = None
             path_key: tuple | None = None
-            css_path_nodes: list[DOMNode] | None = None
 
             if cache is not None and all_pseudo_classes.isdisjoint(
                 self._EXCLUDE_PSEUDO_CLASSES_FROM_CACHE
@@ -660,12 +659,20 @@ def apply(
                     self._process_component_classes(node)
                     return
 
+            # Matching already declares whether a group addresses its target
+            # alone. Read the live declaration only after the first cache miss;
+            # selector lists remain editable and hits need no path planning.
+            requires_path = any(
+                not group.is_compound for rule in rules for group in rule.selector_set
+            )
+            css_path_nodes = node.css_path_nodes if requires_path else [node]
+
+            if cache_key is not None:
                 # Widgets in repeated rows have distinct parent *instances* but
                 # frequently the same selector/pseudo-class ancestry. A CSS rule
                 # cannot distinguish these paths when positional/focus-within
                 # selectors were excluded above. Share resolved rules within this
                 # update batch rather than matching every one from scratch.
-                css_path_nodes = node.css_path_nodes
                 path_key = (
                     "css_path",
                     rule_key,
@@ -681,9 +688,6 @@ def apply(
                     self._process_component_classes(node)
                     return
 
-            if css_path_nodes is None:
-                css_path_nodes = node.css_path_nodes
-
             # Rules that may be set to the special value `initial`
             initial: set[str] = set()
             # Rules in DEFAULT_CSS set to the special value `initial`

tests/css/test_stylesheet.py

@@ -18,6 +18,32 @@
 from textual.widgets import Static
 
 
+def test_target_rule_publication_acquires_only_declared_css_path():
+    class PathReads(DOMNode):
+        path_reads = 0
+
+        @property
+        def css_path_nodes(self):
+            self.path_reads += 1
+            return super().css_path_nodes
+
+    parent = DOMNode(classes="outer")
+    node = PathReads(classes="leaf")
+    node._attach(parent)
+    stylesheet = Stylesheet()
+    stylesheet.add_source(".leaf { color: red; }")
+    stylesheet.apply(node)
+    assert node.styles.color == Color.parse("red")
+    assert node.path_reads == 0
+
+    # Reparse a real child declaration: full ancestry and specificity must
+    # become authoritative again, not the previous target-only key.
+    stylesheet.add_source(".outer > .leaf { color: blue; }")
+    stylesheet.apply(node)
+    assert node.styles.color == Color.parse("blue")
+    assert node.path_reads == 1
+
+
 async def test_equal_current_rules_retarget_a_pending_transition():
     app = App()
     async with app.run_test() as pilot: