test(tui): enhance TDD regression test for #11039

- Verify inherited Widget._render() returns non-None via MRO traversal
- Actually invoke _refresh_display() and verify correct overlay content

Refs: #11039

features/steps/tui_first_run_steps.py

@@ -532,3 +532,55 @@ def step_overlay_refresh_display_callable(context: object) -> None:
     )
     refresh = context._overlay._refresh_display
     assert callable(refresh), "_refresh_display must be callable"
+
+
+@then("the inherited Widget._render should return a proper renderable")
+def step_inherited_render_returns_renderable(context: object) -> None:
+    """Verify the inherited ``Widget._render()`` returns a non-None renderable.
+
+    Bug #11039: ``ActorSelectionOverlay`` defined its own ``_render() -> None``
+    which returned ``None``, causing Textual's layout engine to crash.  The
+    fix renamed it to ``_refresh_display()``.  This assertion confirms that
+    when Textual IS available, calling the real inherited ``_render()``
+    produces a proper renderable (not ``None``).  When Textual is not
+    available (``_FallbackStatic`` has no ``_render``), the check passes
+    trivially since the fallback case cannot trigger the shadowing crash.
+    """
+
+    from cleveragents.tui.widgets.actor_selection_overlay import (
+        ActorSelectionOverlay,
+    )
+
+    cls = ActorSelectionOverlay
+    overlay = context._overlay
+
+    render_from_mro = None
+    for base in cls.__mro__[1:]:
+        if "_render" in base.__dict__:
+            render_from_mro = base.__dict__["_render"]
+            break
+
+    if render_from_mro is None:
+        return
+
+    result = render_from_mro(overlay)
+    assert result is not None, (
+        "Bug #11039 regression: inherited Widget._render() returned None. "
+        "ActorSelectionOverlay may still be shadowing the base-class "
+        "_render method, which would cause a NoneType crash during "
+        "Textual layout in textual >=1.0."
+    )
+
+
+@then("_refresh_display should produce correct overlay content")
+def step_refresh_display_produces_content(context: object) -> None:
+    """Invoke ``_refresh_display()`` and verify it renders correct content."""
+    from typing import cast
+
+    overlay = context._overlay
+    overlay._refresh_display()
+    content: str = cast(str, overlay._text)
+    assert content, "_refresh_display must produce non-empty rendered overlay content"
+    assert "Welcome to CleverAgents" in content, (
+        "_refresh_display content must include the welcome header"
+    )

features/tui_first_run.feature

@@ -206,9 +206,11 @@ Feature: TUI first-run experience with actor selection overlay
   Scenario: ActorSelectionOverlay does not define its own _render method
     Given a new ActorSelectionOverlay
     Then the overlay should not override Textual Widget._render
+    And the inherited Widget._render should return a proper renderable
 
   @tdd_issue @tdd_issue_11039
   Scenario: ActorSelectionOverlay has refresh_display callable instead of render
     Given a new ActorSelectionOverlay
     When I call show on the overlay
     Then the overlay _refresh_display method should be callable
+    And _refresh_display should produce correct overlay content