Check attachment only when a widget has a held layout box

evidence/mutation-box-admission-20261007/README.md

@@ -0,0 +1,45 @@
+# Mutation-box admission
+
+Ordinary Widget box measurements previously called is_attached before asking
+Compositor whether the widget owns a held mutation box. The original retained
+channel cProfile contains about28,000 box queries, nearly all taking that check.
+Its wall/cumulative profile is not a reliable attribution of total UI CPU.
+
+The compositor already owns transient layout_geometry, filled only for the
+acquired mutation roots during original arrangement and restored in finally.
+mutation_box now checks membership first and attachment only for a candidate.
+Widget still uses its actual screen descriptor. NoScreen permits ordinary
+measurement only when is_attached is false; an attached source retains the
+original NoScreen refusal. Other errors propagate. Custom attachment and screen
+behavior stays dynamically dispatched; no snapshot or cached lifetime is added.
+
+Native arrangement, style/child epochs, full and visible maps, hit geometry,
+clipping, mutation exclusions and held-box margins remain unchanged. This removes
+unnecessary attachment traversal from ordinary measurement; it does not reduce
+required scroll placement frames or certify all source invalidations redundant.
+
+Existing Package parsed250 production modules with zero omissions; mutation_box
+has exactly one consumer in Widget._get_box_model. New ordinary/detached sizing
+plus affected held-root geometry/callback publication controls:2PASS/.49s.
+Final attached/custom NoScreen refusal control:1PASS/.36s.
+
+One original unprofiled channel-history driver run retained installed Toad ef421
+and Core d4b9, replacing only native source at e7b105fd4. Native includes ffcb82
+and PreparedTextArea97. Original readonly runtime:
+/home/ts/wt/toad-sidebar-context-pointer-20261006/.artifacts/channel-history-delivery-20261007/runtime
+Driver history_check.py from the same artifact; HISTORY_PROFILE=0 and new owned
+output /home/ts/.cache/agent-scratch/native98-channel-history-20261007.
+
+160 authored initial private messages,20 incoming private messages,30 PageUp;
+no provider, public input, package or installed pin change. Terminal0, empty
+stderr,120 mounted rows/oldest sequence33, no App exception. p95 loop28.59ms,
+maximum134.26ms,38 intervals over50ms. Matching unprofiled baseline p95 28.84ms,
+maximum159.89ms,46 intervals over50ms. One pair establishes no reliable overall
+improvement; meaningful pauses remain. Pilot global-idle waits remain included.
+Original runtime_fixture owns teardown; controller joined and no driver/output
+process reference remained. Exact child birth identities were not recorded.
+
+The subsequent final source change only restores the original attached NoScreen
+refusal, exercised separately above. The normal mounted App path is unchanged;
+no repeat App run was needed. Source-only/author-App evidence, not installed
+physical acceptance. Parent owns durable delivery and affected live observation.

evidence/mutation-box-admission-20261007/app-result.json

@@ -0,0 +1,11 @@
+{
+  "main_loop_samples": 2504,
+  "median_ms": 5.9183514676988125,
+  "p95_ms": 28.594268020242453,
+  "max_ms": 134.25777899101377,
+  "gaps_above_50ms": 38,
+  "mounted_rows": 120,
+  "oldest_sequence": 33,
+  "app_error": null,
+  "profiling": false
+}

src/textual/_compositor.py

@@ -1664,7 +1664,7 @@ def _publication_cuts(self, exclusions: Iterable[Region]) -> list[list[int]]:
 
     def mutation_box(self, widget: Widget) -> BoxModel | None:
         """Borrow the committed root box; an unplaced mutation has no size yet."""
-        if widget not in self._layout_geometry:
+        if widget not in self._layout_geometry or not widget.is_attached:
             return None
         geometry = self._layout_geometry[widget]
         size = Size() if geometry is None else geometry.region.size

src/textual/widget.py

@@ -1945,7 +1945,15 @@ def _get_box_model(
         Returns:
             The size and margin for this widget.
         """
-        if self.is_attached and (box := self.screen._compositor.mutation_box(self)) is not None:
+        try:
+            box = self.screen._compositor.mutation_box(self)
+        except NoScreen:
+            # Unmounted widgets still have ordinary native measurements.
+            # An attached source with no screen retains the original refusal.
+            if self.is_attached:
+                raise
+            box = None
+        if box is not None:
             # Parent layouts measure the same committed box as Compositor;
             # never descend through an actively changing child to size siblings.
             return box

tests/test_mutation_box_admission.py

@@ -0,0 +1,64 @@
+from fractions import Fraction
+
+import pytest
+
+from textual.app import App
+from textual.widget import NoScreen
+from textual.widgets import Static
+
+
+class ObservedAttachment(Static):
+    attachment_reads = 0
+
+    @property
+    def is_attached(self) -> bool:
+        self.attachment_reads += 1
+        return super().is_attached
+
+
+async def test_ordinary_box_measurement_does_not_acquire_mutation_attachment():
+    app = App()
+    async with app.run_test(size=(60, 20)) as pilot:
+        widget = ObservedAttachment("native content")
+        widget.styles.width = 20
+        widget.styles.height = 3
+        await app.mount(widget)
+        await pilot.pause()
+        widget.attachment_reads = 0
+        boxes = [widget._get_box_model(app.size, app.size, Fraction(60), Fraction(20))
+                 for _ in range(20)]
+        assert widget.attachment_reads == 0
+        assert all(box == boxes[0] for box in boxes)
+        assert (boxes[0].width, boxes[0].height) == (20, 3)
+
+        # NoScreen is the original unmounted boundary, not a refusal to measure
+        # an ordinary authored box in this active application.
+        detached = ObservedAttachment("not mounted")
+        detached.styles.width = 7
+        detached.styles.height = 2
+        box = detached._get_box_model(app.size, app.size, Fraction(60), Fraction(20))
+        assert (box.width, box.height) == (7, 2)
+
+
+class RefusedScreen(ObservedAttachment):
+    refuse_screen = False
+
+    @property
+    def screen(self):
+        if self.refuse_screen:
+            raise NoScreen("original custom source refused its screen")
+        return super().screen
+
+
+async def test_attached_custom_screen_refusal_is_not_unmounted_measurement():
+    app = App()
+    async with app.run_test() as pilot:
+        widget = RefusedScreen("attached source")
+        await app.mount(widget)
+        await pilot.pause()
+        widget.refuse_screen = True
+        try:
+            with pytest.raises(NoScreen, match="original custom source"):
+                widget._get_box_model(app.size, app.size, Fraction(80), Fraction(24))
+        finally:
+            widget.refuse_screen = False