test(harness): keep the visual double's signature tied to production

Moves set_scrolling_state's frame_hold into the test double here, where
DisplayManager gains it, rather than in #534 where it arrived a PR early.
CodeRabbit flagged the #534 version correctly: a double that accepts an
argument production does not lets the call pass every harness run and
raise TypeError on the panel, which is the one failure a safety harness
exists to prevent.

The drift has now gone both ways across two branches -- double behind
production on this branch, double ahead of it on #534 -- so it is pinned
instead of remembered. test_display_double_parity.py compares the two
signatures and fails with the direction of the drift named. It reads the
files with ast rather than importing them, because display_manager
imports rgbmatrix at module scope and this check should hold on a laptop
and in CI as well as on a Pi.

Plugins begin passing frame_hold in ledmatrix-plugins#462, which is why
production and the double both need it before that lands.

Full suite: 3889 passed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014RRtqXDCnvnY6EQwhT5CV9

src/plugin_system/testing/visual_display_manager.py

@@ -503,9 +503,18 @@ class VisualTestDisplayManager:
     # Scrolling state (no-op interface compat)
     # ------------------------------------------------------------------
 
-    def set_scrolling_state(self, is_scrolling: bool):
+    def set_scrolling_state(self, is_scrolling: bool, frame_hold: int = 1):
-        """Set the current scrolling state (no-op for testing)."""
+        """Set the current scrolling state (no-op for testing).
+
+        ``frame_hold`` mirrors the DisplayManager signature this change adds.
+        The two are kept in step deliberately: a double that accepts arguments
+        production does not lets a call pass every harness run and then raise
+        TypeError on the panel, and a double that lacks one production has
+        fails every render of a plugin that legitimately paces its scroll.
+        Plugins begin passing it in ledmatrix-plugins#462.
+        """
         self._scrolling_state['is_scrolling'] = is_scrolling
+        self._scrolling_state['frame_hold'] = frame_hold
         if is_scrolling:
             self._scrolling_state['last_scroll_activity'] = time.time()
 

test/test_display_double_parity.py

@@ -0,0 +1,69 @@
+"""The visual-test double must not drift from the real DisplayManager.
+
+Both directions of drift are silent and both are damaging:
+
+  * the double accepts an argument production does not -- the call passes
+    every harness run and raises TypeError on the panel, which is precisely
+    the failure a safety harness exists to prevent;
+  * the double lacks an argument production has -- every plugin that
+    legitimately uses it fails every render, and the harness blames the
+    plugin.
+
+`set_scrolling_state` has been each of those in turn across two branches, so
+the parity is asserted rather than remembered.
+
+Read with ast rather than imported: src/display_manager.py imports rgbmatrix
+at module scope, which is absent anywhere without the panel library, and this
+check should hold on a laptop and in CI as well as on a Pi.
+"""
+
+import ast
+from pathlib import Path
+
+import pytest
+
+ROOT = Path(__file__).resolve().parent.parent
+REAL = ROOT / "src" / "display_manager.py"
+DOUBLE = ROOT / "src" / "plugin_system" / "testing" / "visual_display_manager.py"
+
+#: Methods a plugin calls on whichever manager it is handed.
+SHARED_METHODS = ["set_scrolling_state", "is_currently_scrolling"]
+
+
+def _signature(path: Path, class_hint: str, method: str):
+    """(name, default-repr) pairs for `method`, or None if it is absent."""
+    tree = ast.parse(path.read_text(encoding="utf-8"))
+    for node in ast.walk(tree):
+        if not isinstance(node, ast.ClassDef) or class_hint not in node.name:
+            continue
+        for item in node.body:
+            if isinstance(item, (ast.FunctionDef, ast.AsyncFunctionDef)) and item.name == method:
+                args = [a.arg for a in item.args.args if a.arg != "self"]
+                pad = [None] * (len(args) - len(item.args.defaults))
+                defaults = pad + [ast.unparse(d) for d in item.args.defaults]
+                return list(zip(args, defaults))
+    return None
+
+
+@pytest.mark.parametrize("method", SHARED_METHODS)
+def test_the_double_matches_production(method):
+    real = _signature(REAL, "DisplayManager", method)
+    double = _signature(DOUBLE, "DisplayManager", method)
+
+    assert real is not None, f"DisplayManager lost {method}"
+    assert double is not None, \
+        f"the test double is missing {method}, so every plugin using it fails to render"
+    assert double == real, (
+        f"{method} has drifted: production takes {real}, the double takes {double}. "
+        "A double that is more permissive hides a production TypeError; one that "
+        "is less permissive fails plugins that are actually correct."
+    )
+
+
+def test_frame_hold_is_accepted_by_both():
+    """The specific argument that has drifted twice."""
+    for path, label in ((REAL, "DisplayManager"), (DOUBLE, "VisualTestDisplayManager")):
+        params = dict(_signature(path, "DisplayManager", "set_scrolling_state") or [])
+        assert "frame_hold" in params, f"{label} does not accept frame_hold"
+        assert params["frame_hold"] == "1", \
+            f"{label} must default frame_hold to 1 so existing callers are unaffected"