Declare container selection separately from native box measurement

src/textual/_measurement.py

@@ -44,6 +44,17 @@ def depends(self, widget: Widget, *, greedy: bool = True) -> bool:
         return False
 
 
+class NativeContainerSelection(IndependentHeight):
+    """Native selection reads child membership / layout, not paint rules.
+
+    User-declared independent getters retain IndependentHeight's conservative
+    style contract; height independence alone says nothing about paint inputs.
+    """
+
+    def styles_sensitive(self, widget: Widget) -> bool:
+        return False
+
+
 class StoredVirtualSize(HeightDependency):
     """The original line surface measures its authored reactive extent.
 
@@ -69,6 +80,8 @@ def layout_dependency(self, widget: Widget) -> HeightDependency:
         """The native layout owns the selected measurement's dependency."""
 
     def depends(self, widget: Widget, *, greedy: bool = True) -> bool:
+        if widget._container_selection_dependency.depends(widget):
+            return True
         if not widget.is_container:
             # Native leaf visuals receive rules and width, not container height.
             return False
@@ -79,13 +92,17 @@ def depends(self, widget: Widget, *, greedy: bool = True) -> bool:
         return self.layout_dependency(widget).depends(widget)
 
     def box_depends(self, widget: Widget, *, greedy: bool = True) -> bool:
+        if widget._container_selection_dependency.depends(widget):
+            return True
         if not widget.is_container:
             return False
         if not widget._native_measurement_layout_hooks:
             return True
         return self.layout_dependency(widget).box_depends(widget)
 
     def styles_sensitive(self, widget: Widget) -> bool:
+        if widget._container_selection_dependency.styles_sensitive(widget):
+            return True
         if not widget.is_container:
             return widget._render_styles_sensitive()
         if not widget._native_measurement_layout_hooks:
@@ -213,6 +230,7 @@ def depends(self, widget: Widget, *, greedy: bool = True) -> bool:
 
 CONTEXT_HEIGHT = ContextHeight()
 INDEPENDENT_HEIGHT = IndependentHeight()
+NATIVE_CONTAINER_SELECTION = NativeContainerSelection()
 STORED_VIRTUAL_SIZE = StoredVirtualSize()
 NATIVE_WIDGET_HEIGHT = NativeWidgetHeight()
 NATIVE_WIDGET_WIDTH = NativeWidgetWidth()
@@ -230,6 +248,10 @@ def height_dependency(policy: HeightDependency) -> Callable[[Function], Function
 
     This does not cache a method's result or retain a widget. A declaration must
     describe the whole implementation, including additional work around super().
+    A declared is_container getter must also publish changes to its selection
+    inputs through the original layout invalidation (refresh(layout=True),
+    layout reactives, or native child/style publication). Independence from
+    incoming height does not make independently changing selection immutable.
     """
     def decorate(function: Function) -> Function:
         function._height_dependency = policy  # type: ignore[attr-defined]
@@ -301,7 +323,8 @@ def _local_box_inputs(widget: Widget) -> tuple[bool, bool, bool, bool, bool]:
 
 def box_depends_on_available_height(widget: Widget, *, greedy: bool) -> bool:
     """Conservative proof for the native box resolver, including its extrema."""
-    if not widget._native_box_measurement:
+    if (not widget._native_box_measurement
+            or widget._container_selection_dependency.depends(widget)):
         return True
     depends, content_width, content_height, _, _ = _local_box_inputs(widget)
     if depends:

src/textual/scroll_view.py

@@ -7,7 +7,7 @@
 from rich.console import RenderableType
 
 from textual._animator import EasingFunction
-from textual._measurement import STORED_VIRTUAL_SIZE, height_dependency
+from textual._measurement import NATIVE_CONTAINER_SELECTION, STORED_VIRTUAL_SIZE, height_dependency
 from textual._types import AnimationLevel, CallbackType
 from textual.containers import ScrollableContainer
 from textual.geometry import Region, Size
@@ -39,6 +39,7 @@ def is_scrollable(self) -> bool:
         return True
 
     @property
+    @height_dependency(NATIVE_CONTAINER_SELECTION)
     def is_container(self) -> bool:
         """Since a ScrollView should be a line-api widget, it won't have children,
         and therefore isn't a container."""

src/textual/widget.py

@@ -56,7 +56,8 @@
 from textual._easing import DEFAULT_SCROLL_EASING
 from textual._extrema import Extrema
 from textual._measurement import (
-    CONTEXT_HEIGHT, INDEPENDENT_HEIGHT, NATIVE_WIDGET_HEIGHT, NATIVE_WIDGET_WIDTH,
+    CONTEXT_HEIGHT, INDEPENDENT_HEIGHT, NATIVE_CONTAINER_SELECTION,
+    NATIVE_WIDGET_HEIGHT, NATIVE_WIDGET_WIDTH,
     HeightDependency, _arrangement_wrapper_uses_height, _local_box_inputs,
     box_depends_on_available_height, height_dependency,
 )
@@ -313,6 +314,7 @@ class Widget(DOMNode):
 
     _content_height_dependency: ClassVar[HeightDependency] = NATIVE_WIDGET_HEIGHT
     _content_width_dependency: ClassVar[HeightDependency] = NATIVE_WIDGET_WIDTH
+    _container_selection_dependency: ClassVar[HeightDependency] = NATIVE_CONTAINER_SELECTION
     _native_box_measurement: ClassVar[bool] = True
     _native_measurement_layout_hooks: ClassVar[bool] = True
 
@@ -2693,7 +2695,7 @@ def _relative_children_height(self) -> tuple[bool, bool]:
         guess this separate box-stretch answer.
         """
         native = (
-            type(self).is_container is Widget.is_container
+            not self._container_selection_dependency.depends(self)
             and type(self).children is DOMNode.children
             and type(self.styles) is RenderStyles
         )
@@ -2865,6 +2867,7 @@ def layout(self) -> Layout:
         return self.styles.layout or self._default_layout
 
     @property
+    @height_dependency(NATIVE_CONTAINER_SELECTION)
     def is_container(self) -> bool:
         """Is this widget a container (contains other widgets)?"""
         return bool(self._nodes) or self.styles.layout is not None
@@ -4193,12 +4196,14 @@ def __init_subclass__(
         # on every measurement. Undeclared overrides select the safe full context.
         cls._content_height_dependency = getattr(cls.get_content_height, "_height_dependency", CONTEXT_HEIGHT)
         cls._content_width_dependency = getattr(cls.get_content_width, "_height_dependency", CONTEXT_HEIGHT)
+        cls._container_selection_dependency = getattr(
+            getattr(cls.is_container, "fget", None), "_height_dependency", CONTEXT_HEIGHT
+        )
         cls._native_box_measurement = (
             cls._get_box_model is Widget._get_box_model
             and cls._resolve_extrema is Widget._resolve_extrema
             and cls._has_relative_children_height is Widget._has_relative_children_height
             and cls._has_relative_children_width is Widget._has_relative_children_width
-            and cls.is_container is Widget.is_container
             and cls.layout is Widget.layout
         )
         cls._native_measurement_layout_hooks = (

src/textual/widgets/_static.py

@@ -103,6 +103,7 @@ def _has_native_content_measurement(self) -> bool:
             and type(self).get_content_height is Widget.get_content_height
             and type(self.__visual) is Content
             and self._native_box_measurement
+            and not self._container_selection_dependency.styles_sensitive(self)
             and self._native_measurement_layout_hooks
             and not self.is_container
         )

tests/test_height_independent_measurement.py

@@ -15,6 +15,58 @@
 from textual.widgets import Static
 
 
+async def test_declared_container_selection_publishes_original_measurement_lifetime():
+    from textual._measurement import INDEPENDENT_HEIGHT, NATIVE_WIDGET_HEIGHT, height_dependency
+    from textual.reactive import reactive
+
+    class SelectedContent(Static):
+        show_children = reactive(True, layout=True)
+
+        @property
+        @height_dependency(INDEPENDENT_HEIGHT)
+        def is_container(self):
+            return self.show_children and super().is_container
+
+    class UndeclaredSelection(SelectedContent):
+        @property
+        def is_container(self):
+            return super().is_container
+
+    async with App().run_test() as pilot:
+        selected = SelectedContent("leaf")
+        selected.styles.width = 20
+        selected.styles.height = "auto"
+        child = Static("child")
+        child.styles.height = 7
+        await pilot.app.mount(selected)
+        await selected.mount(child)
+        await pilot.pause()
+        assert selected._native_box_measurement
+        first = box(selected, 10)
+        assert first.height == 7
+        assert box(selected, 30) is first
+        selected.show_children = False
+        leaf = box(selected, 10)
+        assert leaf.height == 1
+        assert leaf is not first
+        assert box(selected, 30) is leaf
+        selected.show_children = True
+        child.styles.height = "1fr"
+        await pilot.pause()
+        assert selected._box_depends_on_available_height()
+        assert selected._has_relative_children_height
+        selected.show_children = False
+        await pilot.pause()
+        assert not selected._has_relative_children_height
+        assert not selected._box_depends_on_available_height()
+
+        unknown = UndeclaredSelection("unknown")
+        assert unknown._native_box_measurement
+        assert unknown._box_depends_on_available_height()
+        assert NATIVE_WIDGET_HEIGHT.depends(unknown)
+        assert NATIVE_WIDGET_HEIGHT.styles_sensitive(selected)
+
+
 async def test_fractional_placement_and_optimal_measurement_keep_distinct_dependencies():
     class HeightReadingWidth(Widget):
         def get_content_width(self, container, viewport):