test(harness): flag a mode that draws nothing without reporting it

The controller skips a mode whose display() returns False and treats
anything else -- including None -- as "content was shown". A mode that
draws nothing and does not return False is therefore never skipped, and
because a mode switch clears the panel first, it sits on a blank screen
for its whole display duration. Two sports plugins shipped exactly that.

The harness rendered those modes and passed them, because it called
display() and discarded the result. Capture it, and warn when a render
produced no lit pixels while claiming content.

Warn-only by default, and deliberately so: a scroll mode's first frame
is legitimately its blank scroll-in buffer, which is 42 of these on the
F1 scoreboard alone. Plugins whose modes are known to draw on their
fixture data can opt into failing via harness.json {"empty_check":
"strict"}, matching how the fill check is staged.

Worth being clear about the limit: this only sees what the fixtures
render. It would not have caught the sports bug, whose fixture seeds
games so the empty path never renders -- that needs the source-level
gate in the plugins repo. What it does catch is the same mistake in any
plugin whose empty state the harness does happen to reach, which is
coverage there was none of before.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Udr6MfaFLUPhX5Fgo67Jf5

scripts/check_plugin.py

@@ -41,6 +41,7 @@ from src.plugin_system.testing.loading import (  # noqa: E402
 )
 from src.plugin_system.testing.harness import (  # noqa: E402
     RenderResult, render_plugin_matrix, compare_to_goldens, write_goldens,
+    check_empty_claimed,
     check_scale_up,
 )
 from src.plugin_system.testing.sizes import (  # noqa: E402
@@ -115,6 +116,11 @@ def check_one(plugin_id: str, search_dirs: List[str], sizes, mock_data: Dict,
     declared = load_manifest(plugin_dir).get("display", {}).get("design_size", {})
     design_size = (int(declared.get("width", 128)), int(declared.get("height", 32)))
     fill_strict = spec.get("fill_check") == "strict"
+    # A mode that renders nothing without returning False is never skipped by
+    # the display controller, so it holds a blank panel for its whole duration.
+    # Warn-only by default: a scroll mode's first frame is legitimately its
+    # blank scroll-in buffer.
+    empty_strict = spec.get("empty_check") == "strict"
 
     # Every run: the base config, plus one per harness.json "variant" —
     # a config overlay with its own golden dir (e.g. adaptive layout mode
@@ -142,6 +148,7 @@ def check_one(plugin_id: str, search_dirs: List[str], sizes, mock_data: Dict,
             compare_to_goldens(results, golden_dir)
 
         check_scale_up(results, design_size=design_size, strict=fill_strict)
+        check_empty_claimed(results, strict=empty_strict)
 
         # Tag variant runs so the report and PNG dumps stay distinguishable.
         if variant_name:
@@ -178,6 +185,9 @@ def print_report(all_results: Dict[str, List[RenderResult]]) -> bool:
                     # warn-only underfill: big panel left mostly empty
                     ex, ey = r.fill_extent
                     detail += f" (fill warn: extent {ex:.0%}x{ey:.0%})"
+                if r.empty_claimed and r.empty_ok is None:
+                    detail += (f" (empty warn: drew nothing but display() returned"
+                               f" {r.display_returned!r}, so the mode is not skipped)")
             else:
                 everything_ok = False
                 if r.error is not None:
@@ -191,6 +201,11 @@ def print_report(all_results: Dict[str, List[RenderResult]]) -> bool:
                     ex, ey = r.fill_extent or (0.0, 0.0)
                     status = "FAIL"
                     detail = f" fill: extent {ex:.0%}x{ey:.0%} below required coverage"
+                elif r.empty_ok is False:
+                    status = "FAIL"
+                    detail = (f" drew nothing but display() returned"
+                              f" {r.display_returned!r}; return False so the"
+                              f" controller skips the mode")
                 else:
                     status, detail = "FAIL", ""
             print(f"  [{status}] {r.size_label:>7}  {r.mode}{detail}")