diff --git a/evidence/container-height-membership-20261007/README.md b/evidence/container-height-membership-20261007/README.md
new file mode 100644
index 0000000000..081743a600
--- /dev/null
+++ b/evidence/container-height-membership-20261007/README.md
@@ -0,0 +1,38 @@
+# Native container membership during height measurement
+
+Native `is_container` is true for either committed child membership or an
+explicit layout. Child membership now answers first, avoiding RenderStyles
+layout resolution on every populated step of a relative-height traversal.
+Empty native child lists still consult the current layout declaration.
+
+This changes only the original Widget property. Subclass property overrides,
+relative percentage/fraction admission, hidden children, auto-size propagation,
+layout policies, measurement generations and reparenting remain unchanged.
+NodeList owns real child membership; RenderStyles owns the independently
+changing layout declaration. Neither answer is stored or inferred from bounds.
+
+The recursive relative-height query remains necessary: even independent
+Stream content measurements can have auto boxes stretched by relative-height
+descendants. Removing that query or treating content independence as box
+independence would change geometry. This patch removes a predicate cost; it
+does not claim to remove descendant traversal or solve burst frame latency.
+
+source.json records existing Package AST declarations and consumers across
+249 native and 288 Toad modules, with zero parse omissions. Runtime replacement
+of the entire is_container property continues to dispatch normally. Private
+replacement of native NodeList/RenderStyles with side-effectful nonconforming
+objects is not a native measurement contract.
+
+64 affected measurement/arrangement/structural-generation checks passed in
+4.33s. The existing loaded source Toad App completed with 568 widgets/10 tabs,
+equal relative-height answers for every widget, unchanged geometry and empty
+stderr. It compares old/current short-circuit order on the same acquired tree.
+Five alternating unprofiled trials of 20 query sweeps had medians 49.73ms and
+47.92ms; trial variation overlaps, so this is not a reliable latency improvement.
+The cProfile files retain instrumentation output; summed recursive cumulative
+values are not an aggregate wall-time result and are not used as speed evidence.
+
+No original session, submitted input, provider, package or public App is used.
+The full recursive descendant query remains the outstanding cost; this patch
+does not justify bypassing it, storing another readiness/dependency answer or
+claiming the original 2.869s writer gap has been solved.
diff --git a/evidence/container-height-membership-20261007/app.stderr.log b/evidence/container-height-membership-20261007/app.stderr.log
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/evidence/container-height-membership-20261007/app.stdout.log b/evidence/container-height-membership-20261007/app.stdout.log
new file mode 100644
index 0000000000..79c3240da5
--- /dev/null
+++ b/evidence/container-height-membership-20261007/app.stdout.log
@@ -0,0 +1,40 @@
+{
+  "scope": "source Toad App recursive height queries, not burst/frame latency",
+  "widgets": 568,
+  "tabs": 10,
+  "sweeps_per_trial": 20,
+  "seconds": {
+    "original": [
+      0.05167253699619323,
+      0.04497357609216124,
+      0.043833934934809804,
+      0.0542515879496932,
+      0.04973278904799372
+    ],
+    "membership_first": [
+      0.0633440789533779,
+      0.04001189791597426,
+      0.04056658991612494,
+      0.048753642942756414,
+      0.04791633493732661
+    ]
+  },
+  "median_seconds": {
+    "original": 0.04973278904799372,
+    "membership_first": 0.04791633493732661
+  },
+  "profiles": {
+    "original": {
+      "is_container_seconds": 0.012723577,
+      "relative_height_seconds": 0.15487751200000002
+    },
+    "membership_first": {
+      "is_container_seconds": 0.013725005,
+      "relative_height_seconds": 0.006692936
+    }
+  },
+  "all_relative_answers_equal": true,
+  "geometry_unchanged": true,
+  "inputs": 0,
+  "providers": 0
+}
diff --git a/evidence/container-height-membership-20261007/controls.log b/evidence/container-height-membership-20261007/controls.log
new file mode 100644
index 0000000000..e3e6b294c7
--- /dev/null
+++ b/evidence/container-height-membership-20261007/controls.log
@@ -0,0 +1,2 @@
+................................................................         [100%]
+64 passed in 4.33s
diff --git a/evidence/container-height-membership-20261007/source.json b/evidence/container-height-membership-20261007/source.json
new file mode 100644
index 0000000000..68862ec80f
--- /dev/null
+++ b/evidence/container-height-membership-20261007/source.json
@@ -0,0 +1,344 @@
+{
+  "native": {
+    "modules": 249,
+    "omissions": [],
+    "sites": [
+      [
+        "src/textual/_compositor.py",
+        982,
+        "consumer",
+        "widget.is_container"
+      ],
+      [
+        "src/textual/_measurement.py",
+        19,
+        "declaration",
+        "box_depends"
+      ],
+      [
+        "src/textual/_measurement.py",
+        54,
+        "declaration",
+        "box_depends"
+      ],
+      [
+        "src/textual/_measurement.py",
+        95,
+        "declaration",
+        "box_depends"
+      ],
+      [
+        "src/textual/_measurement.py",
+        147,
+        "consumer",
+        "HeightDependency.box_depends"
+      ],
+      [
+        "src/textual/_measurement.py",
+        163,
+        "consumer",
+        "HeightDependency.box_depends"
+      ],
+      [
+        "src/textual/_measurement.py",
+        20,
+        "consumer",
+        "widget._has_relative_children_height"
+      ],
+      [
+        "src/textual/_measurement.py",
+        47,
+        "consumer",
+        "widget.is_container"
+      ],
+      [
+        "src/textual/_measurement.py",
+        55,
+        "consumer",
+        "widget.is_container"
+      ],
+      [
+        "src/textual/_measurement.py",
+        59,
+        "consumer",
+        "self.layout_dependency(widget).box_depends"
+      ],
+      [
+        "src/textual/_measurement.py",
+        62,
+        "consumer",
+        "widget.is_container"
+      ],
+      [
+        "src/textual/_measurement.py",
+        79,
+        "consumer",
+        "widget.is_container"
+      ],
+      [
+        "src/textual/_measurement.py",
+        98,
+        "consumer",
+        "widget.layout._arrangement_height_dependency.box_depends"
+      ],
+      [
+        "src/textual/_measurement.py",
+        258,
+        "consumer",
+        "widget._content_height_dependency.box_depends"
+      ],
+      [
+        "src/textual/_measurement.py",
+        134,
+        "consumer",
+        "child._box_depends_on_available_height"
+      ],
+      [
+        "src/textual/await_remove.py",
+        68,
+        "consumer",
+        "node._invalidate_layout"
+      ],
+      [
+        "src/textual/dom.py",
+        152,
+        "declaration",
+        "_style_geometry_updated"
+      ],
+      [
+        "src/textual/dom.py",
+        170,
+        "consumer",
+        "self._style_geometry_updated"
+      ],
+      [
+        "src/textual/dom.py",
+        184,
+        "consumer",
+        "node._style_geometry_updated"
+      ],
+      [
+        "src/textual/screen.py",
+        823,
+        "consumer",
+        "node.is_container"
+      ],
+      [
+        "src/textual/scroll_view.py",
+        41,
+        "declaration",
+        "is_container"
+      ],
+      [
+        "src/textual/selection.py",
+        600,
+        "consumer",
+        "child.is_container"
+      ],
+      [
+        "src/textual/selection.py",
+        570,
+        "consumer",
+        "descendant.is_container"
+      ],
+      [
+        "src/textual/selection.py",
+        369,
+        "consumer",
+        "widget.is_container"
+      ],
+      [
+        "src/textual/visual.py",
+        240,
+        "consumer",
+        "widget.is_container"
+      ],
+      [
+        "src/textual/walk.py",
+        220,
+        "consumer",
+        "node.is_container"
+      ],
+      [
+        "src/textual/widget.py",
+        323,
+        "declaration",
+        "_style_geometry_updated"
+      ],
+      [
+        "src/textual/widget.py",
+        332,
+        "declaration",
+        "_measurement_updated"
+      ],
+      [
+        "src/textual/widget.py",
+        339,
+        "declaration",
+        "_invalidate_layout"
+      ],
+      [
+        "src/textual/widget.py",
+        1904,
+        "declaration",
+        "_box_depends_on_available_height"
+      ],
+      [
+        "src/textual/widget.py",
+        2642,
+        "declaration",
+        "_has_relative_children_height"
+      ],
+      [
+        "src/textual/widget.py",
+        2789,
+        "declaration",
+        "is_container"
+      ],
+      [
+        "src/textual/widget.py",
+        2102,
+        "consumer",
+        "self.is_container"
+      ],
+      [
+        "src/textual/widget.py",
+        2136,
+        "consumer",
+        "self.is_container"
+      ],
+      [
+        "src/textual/widget.py",
+        4692,
+        "consumer",
+        "self.is_container"
+      ],
+      [
+        "src/textual/widget.py",
+        324,
+        "consumer",
+        "super()._style_geometry_updated"
+      ],
+      [
+        "src/textual/widget.py",
+        345,
+        "consumer",
+        "self._measurement_updated"
+      ],
+      [
+        "src/textual/widget.py",
+        1448,
+        "consumer",
+        "self._invalidate_layout"
+      ],
+      [
+        "src/textual/widget.py",
+        2624,
+        "consumer",
+        "self.is_container"
+      ],
+      [
+        "src/textual/widget.py",
+        2645,
+        "consumer",
+        "self.is_container"
+      ],
+      [
+        "src/textual/widget.py",
+        329,
+        "consumer",
+        "self._measurement_updated"
+      ],
+      [
+        "src/textual/widget.py",
+        349,
+        "consumer",
+        "ancestor._measurement_updated"
+      ],
+      [
+        "src/textual/widget.py",
+        1943,
+        "consumer",
+        "self._box_depends_on_available_height"
+      ],
+      [
+        "src/textual/widget.py",
+        4117,
+        "consumer",
+        "cls._has_relative_children_height"
+      ],
+      [
+        "src/textual/widget.py",
+        4117,
+        "consumer",
+        "Widget._has_relative_children_height"
+      ],
+      [
+        "src/textual/widget.py",
+        4119,
+        "consumer",
+        "cls.is_container"
+      ],
+      [
+        "src/textual/widget.py",
+        4119,
+        "consumer",
+        "Widget.is_container"
+      ],
+      [
+        "src/textual/widget.py",
+        4355,
+        "consumer",
+        "self.is_container"
+      ],
+      [
+        "src/textual/widget.py",
+        4594,
+        "consumer",
+        "self._invalidate_layout"
+      ],
+      [
+        "src/textual/widget.py",
+        2059,
+        "consumer",
+        "self._has_relative_children_height"
+      ],
+      [
+        "src/textual/widget.py",
+        2655,
+        "consumer",
+        "child._has_relative_children_height"
+      ],
+      [
+        "src/textual/widgets/_static.py",
+        107,
+        "consumer",
+        "self.is_container"
+      ]
+    ]
+  },
+  "toad": {
+    "modules": 288,
+    "omissions": [],
+    "sites": [
+      [
+        "src/toad/widgets/patch_diff.py",
+        326,
+        "consumer",
+        "code.is_container"
+      ],
+      [
+        "src/toad/widgets/viewport_body.py",
+        506,
+        "declaration",
+        "is_container"
+      ],
+      [
+        "src/toad/widgets/viewport_body.py",
+        510,
+        "consumer",
+        "super().is_container"
+      ]
+    ]
+  }
+}
diff --git a/src/textual/widget.py b/src/textual/widget.py
index 7c212f56da..9fe65c19a4 100644
--- a/src/textual/widget.py
+++ b/src/textual/widget.py
@@ -2788,7 +2788,7 @@ def layout(self) -> Layout:
     @property
     def is_container(self) -> bool:
         """Is this widget a container (contains other widgets)?"""
-        return self.styles.layout is not None or bool(self._nodes)
+        return bool(self._nodes) or self.styles.layout is not None
 
     @property
     def is_scrollable(self) -> bool:
diff --git a/tests/container_height_membership_pilot.py b/tests/container_height_membership_pilot.py
new file mode 100644
index 0000000000..0549a46a53
--- /dev/null
+++ b/tests/container_height_membership_pilot.py
@@ -0,0 +1,89 @@
+"""Measure native relative-height queries on the existing loaded Toad App."""
+
+import asyncio
+import cProfile
+import json
+import os
+from pathlib import Path
+import pstats
+import statistics
+import tempfile
+from time import perf_counter
+
+from agent_comms.comms import wire
+from agent_comms.threads import Thread
+from sidebar_collapse_latency_pilot import FrameApp
+from textual.widget import Widget
+from toad.widgets.agent_response import AgentResponse
+
+
+async def main(output: Path):
+    output.mkdir(parents=True, exist_ok=True)
+    current = Widget.is_container
+    # The original property differs only in short-circuit order. Both versions
+    # use the same actual native NodeList and RenderStyles on this acquired App.
+    original = property(lambda self: self.styles.layout is not None or bool(self._nodes))
+    with tempfile.TemporaryDirectory(prefix="height-membership-", dir=output) as folder:
+        root = Path(folder)
+        os.environ.update(XDG_CONFIG_HOME=str(root / "config"),
+                          XDG_STATE_HOME=str(root / "state"),
+                          XDG_DATA_HOME=str(root / "data"), AGENT_COMMS_ROOT=str(root / "wire"))
+        comms = wire(root / "wire")
+        comms.registry.declare(Thread(root.name, frozenset(), str(root)))
+        for index in range(24):
+            comms.registry.declare(Thread(f"worker-{index:02}", frozenset(), str(root)))
+        app = FrameApp(project_dir=str(root))
+        try:
+            async with app.run_test(size=(120, 42)) as pilot:
+                for _ in range(9):
+                    await app.session_navigation.new(app.session_navigation.default_source)
+                await app.selected_session.conversation.contents.mount(*[
+                    AgentResponse(f"## Response {index}\n\n" + "paragraph\n\n" * 10)
+                    for index in range(12)
+                ])
+                await pilot.pause()
+                widgets = tuple(app.screen.walk_children())
+                expected = tuple(widget._has_relative_children_height for widget in widgets)
+                geometry = tuple(widget.region for widget in widgets)
+                timings = {"original": [], "membership_first": []}
+                profiles = {}
+                for trial in range(6):
+                    for name, declaration in (("original", original), ("membership_first", current))[::1 if trial % 2 == 0 else -1]:
+                        Widget.is_container = declaration
+                        profile = cProfile.Profile() if trial == 0 else None
+                        if profile:
+                            profile.enable()
+                        start = perf_counter()
+                        for _ in range(20):
+                            assert tuple(widget._has_relative_children_height for widget in widgets) == expected
+                        elapsed = perf_counter() - start
+                        if profile:
+                            profile.disable()
+                            profile.dump_stats(str(output / f"{name}.pstats"))
+                            profiles[name] = {
+                                "is_container_seconds": sum(value[2] for (_, _, function), value in pstats.Stats(profile).stats.items()
+                                                            if function in {"is_container", "<lambda>"}),
+                                "relative_height_seconds": sum(value[3] for (_, _, function), value in pstats.Stats(profile).stats.items()
+                                                               if function == "_has_relative_children_height"),
+                            }
+                        else:
+                            timings[name].append(elapsed)
+                Widget.is_container = current
+                await pilot.pause()
+                assert tuple(widget.region for widget in widgets) == geometry
+                assert app._exception is None
+                result = {"scope": "source Toad App recursive height queries, not burst/frame latency",
+                          "widgets": len(widgets), "tabs": 10, "sweeps_per_trial": 20,
+                          "seconds": timings,
+                          "median_seconds": {name: statistics.median(values) for name, values in timings.items()},
+                          "profiles": profiles, "all_relative_answers_equal": True,
+                          "geometry_unchanged": True, "inputs": 0, "providers": 0}
+                (output / "result.json").write_text(json.dumps(result, indent=2) + "\n")
+                print(json.dumps(result, indent=2), flush=True)
+        finally:
+            Widget.is_container = current
+
+
+if __name__ == "__main__":
+    import sys
+    asyncio.run(main(Path(sys.argv[1])))