Add cross-size/cross-screen plugin safety harness (#361)

* feat(testing): add cross-size/cross-screen plugin safety harness

Render every plugin across all supported matrix sizes (64x32, 128x32,
128x64, 256x32) and every declared screen, failing on crashes, content
drawn past the panel edge, or visual drift vs committed golden images.

- BoundsCheckingDisplayManager: oversized-canvas overflow detection
- harness.py: multi-size/multi-screen render engine + golden compare
- scripts/check_plugin.py: CLI (functional+bounds, --out-dir, --update-golden,
  --freeze-time); render_plugin.py refactored onto shared loading helpers
- test/plugins/test_harness.py + test_plugin_matrix.py (parametrized,
  honors per-plugin test/harness.json; skips when no plugins present)
- MockCacheManager.cache_dir so cache-dir-using plugins load headlessly
- .github/workflows/test.yml + docs/plugin-safety-harness.md

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* fix(testing): address PR review feedback on plugin safety harness

- check_plugin: friendly error for non-numeric --sizes; reject non-object
  --config / --mock-data JSON; sanitize plugin mode before using as a
  filename; stop --update-golden from masking crash/overflow failures
- bounds_display_manager: pad the canvas out to the largest supported panel
  (not a fixed 16px) so far-overshoot coordinates are caught, not clipped
- harness: merge config_schema defaults inside render_plugin_matrix; surface
  update() failures as a non-fatal warning + result field instead of a debug
  log; sanitize mode in golden_path
- loading: fail fast when harness.json references a missing mock_data fixture
- mocks: clean up the per-instance temp cache dir via weakref.finalize
- test_plugin_matrix: add a discovery guard that fails when
  LEDMATRIX_REQUIRE_PLUGINS=1 but none found (still skips locally); type hints
- bound test deps with upper version pins for deterministic CI

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* feat(testing): render plugins across arbitrary panel sizes, not a fixed list

Addresses maintainer feedback that there is no canonical set of supported
panel sizes — a build can be any size/configuration (square, 2x2, 4x4, 8x2,
long strips, tall stacks).

- sizes.py: SUPPORTED_SIZES -> DEFAULT_TEST_SIZES (back-compat alias kept),
  reframed as a representative SAMPLE of real panel-grid arrangements rather
  than an authoritative list; add parse_size_token / coerce_sizes /
  resolve_test_sizes helpers
- sizes are now fully overridable: LEDMATRIX_TEST_SIZES env (global, e.g. test
  on your exact hardware) > per-plugin harness.json "sizes" > default sample;
  CLI --sizes unchanged
- bounds_display_manager: pad the canvas to the largest panel IN THE CURRENT
  RUN (via overflow_extent) instead of a hardcoded max, so cross-size overflow
  detection scales to whatever sizes a run uses
- harness: compute per-run extent and thread it into the bounds manager
- tests: arbitrary-shape + size-parsing/precedence coverage
- docs: rewrite "Supported sizes" -> "Sizes: a sample, not a fixed list"

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* fix(testing): fail the harness on non-connectivity update() errors

Addresses the remaining review thread: recording every update() exception as a
non-fatal warning still let a real update() regression pass green as long as
display() survived. Now update() failures are classified — a tolerated set of
connectivity errors (ConnectionError/TimeoutError/socket/ssl/urllib/http/
requests) is recorded non-fatally (expected with no network in CI), while any
other exception is treated as a genuine bug and fails that render.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* ci(security): pin actions to SHAs and disable checkout credential persistence

Addresses the CodeRabbit/zizmor workflow-hardening finding: pin
actions/checkout and actions/setup-python to full commit SHAs and set
persist-credentials: false on checkout to reduce supply-chain and
token-exposure risk.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* fix(testing): validate positive sizes; narrow requests import except

Two review findings:
- sizes.py: parse_size_token / coerce_sizes now reject non-positive
  dimensions (0x32, -64x32) with a clear message instead of passing invalid
  sizes downstream (CodeRabbit).
- harness.py: the optional `requests` import now catches ImportError
  specifically and logs instead of `except Exception: pass`, clearing the
  Codacy medium "Try, Except, Pass" (harness.py L52) and Ruff S110/BLE001.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/test.yml

@@ -0,0 +1,33 @@
+name: Tests
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+jobs:
+  plugin-safety:
+    name: Plugin safety harness + unit tests
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          persist-credentials: false
+
+      - uses: actions/setup-python@0b93645e9fea7318ecaed2b359559ac225c90a2b # v5.3.0
+        with:
+          python-version: "3.12"
+          cache: pip
+
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install -r requirements.txt -r requirements-test.txt
+          pip install RGBMatrixEmulator
+
+      - name: Run harness + visual rendering tests
+        run: |
+          pytest --no-cov \
+            test/plugins/test_harness.py \
+            test/plugins/test_visual_rendering.py \
+            test/plugins/test_plugin_matrix.py

docs/plugin-safety-harness.md

@@ -0,0 +1,136 @@
+# Plugin Safety Harness
+
+Renders a plugin across **every declared screen (mode)** and **a spread of
+matrix sizes**, and fails if any combination crashes, draws past the panel edge,
+or — for plugins that ship golden images — drifts visually. The goal: change a
+plugin without breaking a size or screen you didn't think to test.
+
+## Sizes: a sample, not a fixed list
+
+There is **no fixed set of supported panel sizes** — an RGB matrix build can be
+any width/height and configuration (square, rectangle, 2×2, 4×4, 8×2, long
+strips, tall stacks). Plugins are expected to read dimensions dynamically
+(`self.display_manager.matrix.width/height`) and lay themselves out
+accordingly, so a hardcoded coordinate or unscaled font shows up as a failure
+here.
+
+The harness therefore renders against a **representative sample** that spans the
+axes of variation (`DEFAULT_TEST_SIZES` in `src/plugin_system/testing/sizes.py`),
+not an authoritative list:
+
+Each module is 64×32; entries are real panel-grid arrangements (cols × rows):
+
+| Size    | Grid | Why it's in the sample                     |
+|---------|------|--------------------------------------------|
+| 64×32   | 1×1  | single panel — tightest common rectangle   |
+| 128×32  | 2×1  | the baseline most plugins are tuned for    |
+| 64×64   | 1×2  | stacked — tall-narrow centering            |
+| 128×64  | 2×2  | block — icon scaling / vertical centering  |
+| 256×32  | 4×1  | long strip — wide horizontal layout        |
+| 128×96  | 2×3  | tall — vertical overflow                   |
+| 256×128 | 4×4  | large block — both dimensions big at once  |
+
+**Override the sizes entirely** to test your actual hardware (or any shape):
+
+```bash
+# CLI — one-off:
+python scripts/check_plugin.py --plugin clock-simple --sizes 8x16,64x64,256x32
+
+# pytest — force every plugin onto your panel(s):
+LEDMATRIX_TEST_SIZES="8x16,128x128" pytest test/plugins/test_plugin_matrix.py
+
+# Per-plugin — declare the shapes a plugin targets in its test/harness.json:
+#   { "sizes": [[8, 16], [64, 64]] }
+```
+
+Precedence: `LEDMATRIX_TEST_SIZES` env (global) → per-plugin `harness.json`
+`sizes` → the default sample. Bounds checking adapts to whatever sizes a run
+uses — the backing canvas is padded out to the **largest** panel in the run, so
+a coordinate meant for a big build is still caught when rendering a small one.
+
+## Quick start
+
+```bash
+# Functional + bounds check across all sizes/screens:
+python scripts/check_plugin.py --plugin clock-simple
+
+# Every discovered plugin:
+python scripts/check_plugin.py --all
+
+# Dump PNGs to eyeball each size/screen:
+python scripts/check_plugin.py --plugin ledmatrix-weather --out-dir /tmp/preview
+```
+
+Exit code is non-zero if any `(plugin, size, screen)` fails. Plugins are
+discovered in `plugin-repos/` and `plugins/` (override with `--plugin-dir`).
+
+## What it checks (Phase 1 — always on)
+
+1. **Loads** and builds its mode list.
+2. **Renders every screen** at every size without raising. `update()` may fail
+   (no network in CI) and is tolerated; a crash in `display()` is a failure —
+   `display()` must handle the no-data state.
+3. **Bounds**: nothing is drawn past the right/bottom edge. Implemented by
+   `BoundsCheckingDisplayManager`, which backs the declared panel with an
+   oversized canvas and flags any pixels that land in the margin. (Left/top
+   overflow at negative coordinates and BDF text are not flagged — golden images
+   cover those.)
+
+## Golden images (Phase 2 — opt-in per plugin)
+
+A plugin opts in by committing reference PNGs and (usually) a small harness spec:
+
+```
+plugins/<id>/test/harness.json          # how to render deterministically
+plugins/<id>/test/fixtures/mock.json     # optional cached data
+plugins/<id>/test/golden/<WxH>/<mode>.png
+```
+
+`test/harness.json` keys (all optional):
+
+```json
+{
+  "config":      { "timezone": "UTC" },
+  "mock_data":   "fixtures/mock.json",
+  "freeze_time": "2025-08-01 15:25:00",
+  "skip_update": false,
+  "sizes":       [[128, 32], [128, 64]]
+}
+```
+
+Generate / refresh goldens after an intentional visual change, then review the
+diff before committing:
+
+```bash
+python scripts/check_plugin.py --plugin clock-simple --update-golden \
+  --config '{"timezone":"UTC"}' --freeze-time "2025-08-01 15:25:00"
+```
+
+Comparison is exact by default (`compare_images` in `harness.py` accepts a
+tolerance for known anti-aliasing noise). Determinism requires a pinned Pillow
+and the bundled fonts — keep both stable when regenerating goldens.
+
+## Tests & CI
+
+- `test/plugins/test_harness.py` — unit tests for bounds detection, image
+  comparison, and mode enumeration (run anywhere).
+- `test/plugins/test_plugin_matrix.py` — parametrized over discovered plugins ×
+  sizes × screens; honors each plugin's `test/harness.json` and goldens. Skips
+  when no plugins are present (e.g. a fresh core checkout); set
+  `LEDMATRIX_REQUIRE_PLUGINS=1` in a pipeline where plugins must be present to
+  turn an empty discovery into a hard failure instead. Point it at the monorepo
+  with `LEDMATRIX_PLUGINS_DIR=/path/to/ledmatrix-plugins/plugins`.
+- `.github/workflows/test.yml` — runs the harness + visual tests on every PR.
+
+The plugin monorepo has its own `Plugin Safety` workflow that runs this harness
+against changed plugins on every PR.
+
+## Developer workflow
+
+1. Change the plugin on a branch.
+2. `python scripts/check_plugin.py --plugin <id> --out-dir /tmp/preview` and
+   eyeball the PNGs.
+3. Intentional visual change? `--update-golden`, review diffs, commit goldens.
+4. (Monorepo) bump `manifest.json` version and let the pre-commit hook sync
+   `plugins.json`.
+5. Push — CI re-runs the harness across all sizes and gates the PR.

requirements-test.txt

@@ -0,0 +1,8 @@
+# Test-only dependencies for the plugin safety harness and pytest suite.
+# Install alongside requirements.txt:  pip install -r requirements.txt -r requirements-test.txt
+# Upper bounds pin the major version so a new release can't silently change
+# golden-image / time-sensitive test behavior between CI runs.
+pytest>=7.4,<9
+pytest-cov>=4.1,<7
+jsonschema>=4.0,<5       # manifest validation
+freezegun>=1.2,<2        # deterministic time for golden-image tests

scripts/check_plugin.py

@@ -0,0 +1,217 @@
+#!/usr/bin/env python3
+"""
+Plugin safety checker.
+
+Renders a plugin across every declared screen (mode) and every supported matrix
+size, and fails if any screen crashes, overflows the panel, or (for plugins with
+committed golden images) drifts visually.
+
+Usage:
+    # Functional + bounds check across all sizes/modes:
+    python scripts/check_plugin.py --plugin clock-simple
+
+    # Every discovered plugin:
+    python scripts/check_plugin.py --all
+
+    # Dump PNGs for each size/mode so you can eyeball them:
+    python scripts/check_plugin.py --plugin ledmatrix-weather --out-dir /tmp/preview
+
+    # Refresh committed golden images after an intentional visual change:
+    python scripts/check_plugin.py --plugin clock-simple --update-golden \
+        --mock-data plugins/clock-simple/test/fixtures/mock.json
+
+Exit code is non-zero if any (plugin, size, mode) fails.
+"""
+
+import argparse
+import json
+import os
+import sys
+from pathlib import Path
+from typing import Dict, List, Optional
+
+PROJECT_ROOT = Path(__file__).resolve().parent.parent
+sys.path.insert(0, str(PROJECT_ROOT))
+
+os.environ['EMULATOR'] = 'true'
+
+from src.logging_config import get_logger  # noqa: E402
+from src.plugin_system.testing.loading import (  # noqa: E402
+    find_plugin_dir, load_config_defaults,
+)
+from src.plugin_system.testing.harness import (  # noqa: E402
+    RenderResult, render_plugin_matrix, compare_to_goldens, write_goldens,
+)
+from src.plugin_system.testing.sizes import (  # noqa: E402
+    DEFAULT_TEST_SIZES, parse_size_token, safe_mode_filename, size_label,
+)
+
+logger = get_logger("[Check Plugin]")
+
+DEFAULT_SEARCH_DIRS = [
+    str(PROJECT_ROOT / 'plugins'),
+    str(PROJECT_ROOT / 'plugin-repos'),
+]
+
+
+def discover_plugins(search_dirs: List[str]) -> List[str]:
+    """All plugin ids found across the search dirs (dirs containing manifest.json)."""
+    found = []
+    for d in search_dirs:
+        base = Path(d)
+        if not base.exists():
+            continue
+        for child in sorted(base.iterdir()):
+            if (child / 'manifest.json').exists() and child.name not in found:
+                found.append(child.name)
+    return found
+
+
+def parse_sizes(spec: Optional[str]):
+    if not spec:
+        return DEFAULT_TEST_SIZES
+    sizes = []
+    for token in spec.split(','):
+        if not token.strip():
+            continue
+        try:
+            sizes.append(parse_size_token(token))
+        except ValueError as exc:
+            raise SystemExit(str(exc)) from exc
+    return sizes
+
+
+def check_one(plugin_id: str, search_dirs: List[str], sizes, mock_data: Dict,
+              config: Dict, run_update: bool, out_dir: Optional[Path],
+              update_golden: bool, golden_dir_override: Optional[Path],
+              freeze_time: Optional[str]) -> List[RenderResult]:
+    plugin_dir = find_plugin_dir(plugin_id, search_dirs)
+    if not plugin_dir:
+        logger.error("Plugin '%s' not found in: %s", plugin_id, search_dirs)
+        return [RenderResult(plugin_id, 0, 0, "<not-found>", error="plugin directory not found")]
+
+    # Start from config_schema defaults so plugins behave like a real install.
+    full_config = {"enabled": True}
+    full_config.update(load_config_defaults(plugin_dir))
+    full_config.update(config)
+
+    results = render_plugin_matrix(
+        plugin_id=plugin_id, plugin_dir=plugin_dir, config=full_config,
+        mock_data=mock_data, sizes=sizes, run_update=run_update,
+        freeze_time=freeze_time,
+    )
+
+    golden_dir = golden_dir_override or (plugin_dir / 'test' / 'golden')
+    if update_golden:
+        written = write_goldens(results, golden_dir)
+        logger.info("Wrote %d golden image(s) for %s to %s", written, plugin_id, golden_dir)
+    else:
+        compare_to_goldens(results, golden_dir)
+
+    if out_dir:
+        for r in results:
+            if r.image is None:
+                continue
+            dest = out_dir / plugin_id / size_label(r.width, r.height)
+            dest.mkdir(parents=True, exist_ok=True)
+            r.image.save(dest / f"{safe_mode_filename(r.mode)}.png", format="PNG")
+
+    return results
+
+
+def print_report(all_results: Dict[str, List[RenderResult]]) -> bool:
+    """Print a per-plugin grid. Returns True if everything passed."""
+    everything_ok = True
+    for plugin_id, results in all_results.items():
+        print(f"\n=== {plugin_id} ===")
+        for r in results:
+            if r.ok:
+                status = "PASS"
+                detail = ""
+                if r.golden_checked:
+                    detail = " (golden ✓)"
+                if r.update_error is not None:
+                    detail += f" (update warn: {r.update_error})"
+            else:
+                everything_ok = False
+                if r.error is not None:
+                    status, detail = "FAIL", f" error={r.error}"
+                elif r.overflow is not None:
+                    status, detail = "FAIL", f" overflow bbox={r.overflow}"
+                elif r.golden_ok is False:
+                    status = "FAIL"
+                    detail = f" golden drift: {r.golden_diff_pixels}px (max Δ={r.golden_max_delta})"
+                else:
+                    status, detail = "FAIL", ""
+            print(f"  [{status}] {r.size_label:>7}  {r.mode}{detail}")
+    print()
+    return everything_ok
+
+
+def main() -> int:
+    parser = argparse.ArgumentParser(description="Check a plugin renders safely across sizes & screens")
+    group = parser.add_mutually_exclusive_group(required=True)
+    group.add_argument('--plugin', '-p', help='Plugin id to check')
+    group.add_argument('--all', action='store_true', help='Check every discovered plugin')
+    parser.add_argument('--plugin-dir', '-d', default=None, help='Directory to search for plugins')
+    parser.add_argument('--sizes', default=None, help='Comma-separated WxH list (default: all supported)')
+    parser.add_argument('--config', '-c', default='{}', help='Plugin config overrides as JSON')
+    parser.add_argument('--mock-data', '-m', default=None, help='Path to JSON file with mock cache data')
+    parser.add_argument('--out-dir', '-o', default=None, help='Also dump rendered PNGs here')
+    parser.add_argument('--skip-update', action='store_true', help='Skip calling update()')
+    parser.add_argument('--update-golden', action='store_true', help='Write/refresh golden images')
+    parser.add_argument('--golden-dir', default=None, help='Override golden dir (default: <plugin>/test/golden)')
+    parser.add_argument('--freeze-time', default=None,
+                        help='Freeze wall clock, e.g. "2025-08-01 15:25:00" (for time-dependent plugins)')
+    args = parser.parse_args()
+
+    search_dirs = [args.plugin_dir] if args.plugin_dir else DEFAULT_SEARCH_DIRS
+    sizes = parse_sizes(args.sizes)
+
+    try:
+        config = json.loads(args.config)
+    except json.JSONDecodeError as e:
+        logger.error("Invalid --config JSON: %s", e)
+        return 2
+    if not isinstance(config, dict):
+        logger.error("--config must be a JSON object, got %s", type(config).__name__)
+        return 2
+
+    mock_data = {}
+    if args.mock_data:
+        mock_path = Path(args.mock_data)
+        if not mock_path.exists():
+            logger.error("Mock data file not found: %s", args.mock_data)
+            return 2
+        with open(mock_path) as f:
+            mock_data = json.load(f)
+        if not isinstance(mock_data, dict):
+            logger.error("--mock-data must be a JSON object (key -> cache value), got %s",
+                         type(mock_data).__name__)
+            return 2
+
+    plugin_ids = discover_plugins(search_dirs) if args.all else [args.plugin]
+    if not plugin_ids:
+        logger.error("No plugins found in: %s", search_dirs)
+        return 2
+
+    out_dir = Path(args.out_dir) if args.out_dir else None
+    golden_dir_override = Path(args.golden_dir) if args.golden_dir else None
+
+    all_results: Dict[str, List[RenderResult]] = {}
+    for plugin_id in plugin_ids:
+        all_results[plugin_id] = check_one(
+            plugin_id=plugin_id, search_dirs=search_dirs, sizes=sizes,
+            mock_data=mock_data, config=config, run_update=not args.skip_update,
+            out_dir=out_dir, update_golden=args.update_golden,
+            golden_dir_override=golden_dir_override, freeze_time=args.freeze_time,
+        )
+
+    # When refreshing goldens we skip drift comparison, but a crash or overflow
+    # still means the plugin is broken — never let --update-golden mask that.
+    ok = print_report(all_results)
+    return 0 if ok else 1
+
+
+if __name__ == '__main__':
+    sys.exit(main())

scripts/render_plugin.py

@@ -17,7 +17,6 @@ import os
 import json
 import argparse
 from pathlib import Path
-from typing import Any, Dict, Optional, Sequence, Union
 
 # Add project root to path
 PROJECT_ROOT = Path(__file__).resolve().parent.parent
@@ -28,49 +27,15 @@ os.environ['EMULATOR'] = 'true'
 
 # Import logger after path setup so src.logging_config is importable
 from src.logging_config import get_logger  # noqa: E402
+from src.plugin_system.testing.loading import (  # noqa: E402
+    find_plugin_dir, load_manifest, load_config_defaults,
+)
 logger = get_logger("[Render Plugin]")
 
 MIN_DIMENSION = 1
 MAX_DIMENSION = 512
 
 
-def find_plugin_dir(plugin_id: str, search_dirs: Sequence[Union[str, Path]]) -> Optional[Path]:
-    """Find a plugin directory by searching multiple paths."""
-    from src.plugin_system.plugin_loader import PluginLoader
-    loader = PluginLoader()
-    for search_dir in search_dirs:
-        search_path = Path(search_dir)
-        if not search_path.exists():
-            continue
-        result = loader.find_plugin_directory(plugin_id, search_path)
-        if result:
-            return Path(result)
-    return None
-
-
-def load_manifest(plugin_dir: Path) -> Dict[str, Any]:
-    """Load and return manifest.json from plugin directory."""
-    manifest_path = plugin_dir / 'manifest.json'
-    if not manifest_path.exists():
-        raise FileNotFoundError(f"No manifest.json in {plugin_dir}")
-    with open(manifest_path, 'r') as f:
-        return json.load(f)
-
-
-def load_config_defaults(plugin_dir: Path) -> Dict[str, Any]:
-    """Extract default values from config_schema.json."""
-    schema_path = plugin_dir / 'config_schema.json'
-    if not schema_path.exists():
-        return {}
-    with open(schema_path, 'r') as f:
-        schema = json.load(f)
-    defaults: Dict[str, Any] = {}
-    for key, prop in schema.get('properties', {}).items():
-        if 'default' in prop:
-            defaults[key] = prop['default']
-    return defaults
-
-
 def main() -> int:
     """Load a plugin, call update() + display(), and save the result as a PNG image."""
     parser = argparse.ArgumentParser(description='Render a plugin display to a PNG image')

src/plugin_system/testing/__init__.py

@@ -7,13 +7,22 @@ Provides base classes and utilities for testing LEDMatrix plugins.
 from .plugin_test_base import PluginTestCase
 from .mocks import MockDisplayManager, MockCacheManager, MockConfigManager, MockPluginManager
 from .visual_display_manager import VisualTestDisplayManager
+from .bounds_display_manager import BoundsCheckingDisplayManager
+from .sizes import (
+    DEFAULT_TEST_SIZES, SUPPORTED_SIZES, resolve_test_sizes, size_label,
+)
 
 __all__ = [
     'PluginTestCase',
     'VisualTestDisplayManager',
+    'BoundsCheckingDisplayManager',
     'MockDisplayManager',
     'MockCacheManager',
     'MockConfigManager',
     'MockPluginManager',
+    'DEFAULT_TEST_SIZES',
+    'SUPPORTED_SIZES',
+    'resolve_test_sizes',
+    'size_label',
 ]
 

src/plugin_system/testing/bounds_display_manager.py

@@ -0,0 +1,129 @@
+"""
+Bounds-checking display manager.
+
+A VisualTestDisplayManager that draws onto an oversized canvas (the declared
+panel size plus a right/bottom margin) while still reporting the declared size
+to the plugin. Content that a plugin draws past the right or bottom edge lands
+in the margin instead of being silently clipped by PIL, so the harness can
+detect overflow — the classic symptom of hardcoded coordinates or fonts/icons
+that don't scale down to a smaller panel.
+
+Limitations (documented on purpose):
+- Overflow past the LEFT or TOP edge (negative coordinates) is still clipped by
+  PIL and not detected here. The dominant real-world breakage is content that is
+  too wide/tall for a smaller panel, which this catches.
+- BDF text is clipped to the declared bounds by the parent's bitmap drawer, so
+  BDF overflow is not flagged. Golden-image regression covers those plugins.
+- If a plugin replaces the canvas with its own image (display_manager.image = ...),
+  the margin can't be measured and overflow is reported as undetermined (None).
+"""
+
+from typing import Optional, Tuple
+
+from .sizes import DEFAULT_TEST_SIZES
+from .visual_display_manager import VisualTestDisplayManager, _MatrixProxy
+
+# Smallest extra band kept on the right/bottom so a few pixels of overflow are
+# still visible even on the largest panel in a run.
+_BASE_MARGIN = 16
+# Fallback overflow reference when a caller doesn't pass one: the largest shape
+# in the default sample. We extend every (smaller) canvas out to at least this
+# size so content drawn at a coordinate meant for a bigger build — e.g. x=200 on
+# a 64-wide panel — lands in the padded region and is flagged, instead of being
+# clipped off-canvas and read as a false pass.
+_DEFAULT_EXTENT_WIDTH = max(w for w, _ in DEFAULT_TEST_SIZES)
+_DEFAULT_EXTENT_HEIGHT = max(h for _, h in DEFAULT_TEST_SIZES)
+
+
+class BoundsCheckingDisplayManager(VisualTestDisplayManager):
+    """Detects drawing that overflows the declared panel size."""
+
+    # Kept for backwards compatibility; real padding is computed per-axis below.
+    MARGIN = _BASE_MARGIN
+
+    def __init__(self, width: int = 128, height: int = 32,
+                 overflow_extent: Optional[Tuple[int, int]] = None):
+        self._declared_width = int(width)
+        self._declared_height = int(height)
+        # Pad the canvas out to at least `overflow_extent` (the largest panel
+        # this run cares about) plus a base margin, so coordinates meant for a
+        # bigger build are caught — not clipped — when rendering a smaller panel.
+        # Defaults to the largest shape in the sample when no run is known.
+        ext_w, ext_h = overflow_extent or (_DEFAULT_EXTENT_WIDTH, _DEFAULT_EXTENT_HEIGHT)
+        self._canvas_width = max(self._declared_width, int(ext_w)) + _BASE_MARGIN
+        self._canvas_height = max(self._declared_height, int(ext_h)) + _BASE_MARGIN
+        # Parent builds the (oversized) backing canvas + fonts.
+        super().__init__(self._canvas_width, self._canvas_height)
+        # Plugins must see the DECLARED size, not the padded canvas size.
+        self.matrix = _MatrixProxy(self._declared_width, self._declared_height)
+
+    # -- declared dimensions (override parent's image-derived properties) --
+
+    @property
+    def width(self) -> int:
+        return self._declared_width
+
+    @property
+    def height(self) -> int:
+        return self._declared_height
+
+    @property
+    def display_width(self) -> int:
+        return self._declared_width
+
+    @property
+    def display_height(self) -> int:
+        return self._declared_height
+
+    # -- overflow detection --
+
+    def _canvas_is_padded(self) -> bool:
+        return self.image.size == (self._canvas_width, self._canvas_height)
+
+    def check_overflow(self) -> Optional[Tuple[int, int, int, int]]:
+        """Bounding box (in full-canvas coords) of any drawing beyond the
+        declared panel, or None if nothing overflowed / undetermined."""
+        if not self._canvas_is_padded():
+            return None
+
+        exp_w = self._canvas_width
+        exp_h = self._canvas_height
+        boxes = []
+
+        right = self.image.crop((self._declared_width, 0, exp_w, exp_h)).getbbox()
+        if right:
+            boxes.append((right[0] + self._declared_width, right[1],
+                          right[2] + self._declared_width, right[3]))
+
+        bottom = self.image.crop((0, self._declared_height, exp_w, exp_h)).getbbox()
+        if bottom:
+            boxes.append((bottom[0], bottom[1] + self._declared_height,
+                          bottom[2], bottom[3] + self._declared_height))
+
+        if not boxes:
+            return None
+        return (
+            min(b[0] for b in boxes), min(b[1] for b in boxes),
+            max(b[2] for b in boxes), max(b[3] for b in boxes),
+        )
+
+    # -- snapshot/image accessors return the cropped, true-panel image --
+
+    def declared_image(self):
+        """The visible panel: the canvas cropped to the declared size."""
+        if self._canvas_is_padded():
+            return self.image.crop((0, 0, self._declared_width, self._declared_height))
+        return self.image
+
+    def save_snapshot(self, path: str) -> None:
+        self.declared_image().save(path, format='PNG')
+
+    def get_image(self):
+        return self.declared_image()
+
+    def get_image_base64(self) -> str:
+        import base64
+        import io
+        buffer = io.BytesIO()
+        self.declared_image().save(buffer, format='PNG')
+        return base64.b64encode(buffer.getvalue()).decode('utf-8')

src/plugin_system/testing/harness.py

@@ -0,0 +1,314 @@
+"""
+Plugin safety harness.
+
+Renders a plugin across every declared screen (mode) and every supported matrix
+size, capturing crashes and overflow. Used by scripts/check_plugin.py and the
+pytest matrix test to guarantee a plugin change doesn't break a screen at a size
+the author didn't try.
+
+The render flow mirrors scripts/render_plugin.py (same PluginLoader call), but
+this module adds: multi-size iteration, per-mode rendering, overflow detection
+via BoundsCheckingDisplayManager, and golden-image comparison.
+"""
+
+import contextlib
+import http.client
+import inspect
+import socket
+import ssl
+import urllib.error
+from dataclasses import dataclass
+from pathlib import Path
+from typing import Any, Dict, List, Optional, Tuple
+
+from PIL import Image, ImageChops
+
+from src.logging_config import get_logger
+from .bounds_display_manager import BoundsCheckingDisplayManager
+from .loading import load_config_defaults, load_manifest
+from .sizes import DEFAULT_TEST_SIZES, safe_mode_filename, size_label
+
+logger = get_logger("[Plugin Harness]")
+
+
+def _tolerated_update_errors() -> Tuple[type, ...]:
+    """Exception types from update() we treat as a tolerated no-connectivity
+    failure (expected in CI / headless dev) rather than a real plugin bug.
+
+    Anything NOT in this set is a genuine regression — a plugin that lets a
+    non-network exception escape update() should fail the harness, not pass
+    green because display() happened to survive.
+    """
+    types: List[type] = [
+        ConnectionError, TimeoutError,        # builtins
+        socket.gaierror, socket.timeout,      # DNS / socket timeouts
+        ssl.SSLError,
+        urllib.error.URLError,
+        http.client.HTTPException,
+    ]
+    try:  # requests is optional; cover its whole error tree when present
+        import requests
+        types.append(requests.exceptions.RequestException)
+    except ImportError:  # pragma: no cover - requests not installed
+        logger.debug("requests not installed; its connectivity errors won't be specifically tolerated")
+    return tuple(types)
+
+
+_TOLERATED_UPDATE_ERRORS = _tolerated_update_errors()
+
+
+@dataclass
+class RenderResult:
+    """Outcome of rendering one (size, mode) of a plugin."""
+    plugin_id: str
+    width: int
+    height: int
+    mode: str
+    image: Optional[Image.Image] = None
+    error: Optional[str] = None          # fatal: load/display crash, or a non-network update() error
+    update_error: Optional[str] = None   # tolerated: connectivity error from update() (no network in CI)
+    overflow: Optional[Tuple[int, int, int, int]] = None  # bbox past the panel
+    # golden comparison (populated only when a golden was provided)
+    golden_checked: bool = False
+    golden_ok: Optional[bool] = None
+    golden_diff_pixels: int = 0
+    golden_max_delta: int = 0
+
+    @property
+    def size_label(self) -> str:
+        return size_label(self.width, self.height)
+
+    @property
+    def ok(self) -> bool:
+        """Phase-1 pass: rendered without crashing and without overflow, and if a
+        golden was checked it matched."""
+        if self.error is not None or self.overflow is not None:
+            return False
+        if self.golden_checked and self.golden_ok is False:
+            return False
+        return True
+
+
+def list_modes(plugin_instance: Any, manifest: Dict[str, Any], plugin_id: str) -> List[str]:
+    """Enumerate a plugin's screens: instance.modes wins, then manifest
+    display_modes, then the plugin id as a single mode."""
+    modes = getattr(plugin_instance, "modes", None)
+    if modes:
+        return [str(m) for m in modes]
+    declared = manifest.get("display_modes")
+    if declared:
+        return [str(m) for m in declared]
+    return [plugin_id]
+
+
+def _instantiate(plugin_id: str, manifest: Dict[str, Any], plugin_dir: Path,
+                 config: Dict[str, Any], mock_data: Dict[str, Any],
+                 display_manager: Any) -> Any:
+    """Load and construct a plugin instance with mocked managers."""
+    from src.plugin_system.plugin_loader import PluginLoader
+    from src.plugin_system.testing import MockCacheManager, MockPluginManager
+
+    cache_manager = MockCacheManager()
+    for key, value in (mock_data or {}).items():
+        cache_manager.set(key, value)
+
+    loader = PluginLoader()
+    plugin_instance, _module = loader.load_plugin(
+        plugin_id=plugin_id,
+        manifest=manifest,
+        plugin_dir=plugin_dir,
+        config=config,
+        display_manager=display_manager,
+        cache_manager=cache_manager,
+        plugin_manager=MockPluginManager(),
+        install_deps=False,
+    )
+    return plugin_instance
+
+
+def _render_mode(plugin_instance: Any, mode: str) -> None:
+    """Render a specific screen. Prefer an explicit display_mode kwarg; otherwise
+    drive the plugin's internal mode state machine (first display() call renders
+    modes[current_mode_index] when current_display_mode is None)."""
+    sig = inspect.signature(plugin_instance.display)
+    if "display_mode" in sig.parameters:
+        plugin_instance.display(force_clear=True, display_mode=mode)
+        return
+
+    modes = getattr(plugin_instance, "modes", None)
+    if modes and mode in modes:
+        plugin_instance.current_mode_index = list(modes).index(mode)
+    if hasattr(plugin_instance, "current_display_mode"):
+        plugin_instance.current_display_mode = None
+    plugin_instance.display(force_clear=False)
+
+
+def _freeze(freeze_time: Optional[str]):
+    """Context manager that freezes wall-clock time when freeze_time is given,
+    so time-dependent plugins (clocks, countdowns) render deterministic goldens."""
+    if not freeze_time:
+        return contextlib.nullcontext()
+    try:
+        from freezegun import freeze_time as _ft
+    except ImportError as e:  # pragma: no cover - only hit without the dep
+        raise RuntimeError(
+            "freeze_time requires the 'freezegun' package (pip install freezegun)"
+        ) from e
+    return _ft(freeze_time)
+
+
+def render_plugin_matrix(
+    plugin_id: str,
+    plugin_dir: Path,
+    config: Optional[Dict[str, Any]] = None,
+    mock_data: Optional[Dict[str, Any]] = None,
+    sizes: Optional[List[Tuple[int, int]]] = None,
+    run_update: bool = True,
+    freeze_time: Optional[str] = None,
+) -> List[RenderResult]:
+    """Render every (size, mode) combination for a plugin.
+
+    Returns a flat list of RenderResult. A fresh plugin instance is built per
+    (size, mode) so state never leaks between screens. Pass freeze_time (e.g.
+    "2025-08-01 15:25:00") to make time-dependent plugins reproducible.
+    """
+    plugin_dir = Path(plugin_dir)
+    manifest = load_manifest(plugin_dir)
+    # Start from config_schema.json defaults so the plugin behaves like a real
+    # install; explicit caller config still wins over a schema default.
+    config = {"enabled": True, **load_config_defaults(plugin_dir), **(config or {})}
+    sizes = sizes or DEFAULT_TEST_SIZES
+    results: List[RenderResult] = []
+
+    # The largest panel in this run. Every (smaller) canvas is padded out to it
+    # so a coordinate meant for the biggest configuration is still caught when
+    # rendering a smaller one, instead of being clipped into a false pass.
+    extent = (max(w for w, _ in sizes), max(h for _, h in sizes))
+
+    with _freeze(freeze_time):
+        for width, height in sizes:
+            results.extend(_render_size(
+                plugin_id, manifest, plugin_dir, config, mock_data or {},
+                width, height, run_update, extent,
+            ))
+
+    return results
+
+
+def _render_size(plugin_id, manifest, plugin_dir, config, mock_data,
+                 width, height, run_update, extent) -> List[RenderResult]:
+    """Render every mode at one size. A fresh instance per mode avoids state leaks."""
+    results: List[RenderResult] = []
+
+    # Discover modes once per size (instance build can depend on config).
+    try:
+        probe_dm = BoundsCheckingDisplayManager(width=width, height=height, overflow_extent=extent)
+        probe = _instantiate(plugin_id, manifest, plugin_dir, config, mock_data, probe_dm)
+        modes = list_modes(probe, manifest, plugin_id)
+    except Exception as e:  # noqa: BLE001 — surface any load failure as a result
+        return [RenderResult(plugin_id, width, height, "<load>", error=repr(e))]
+
+    for mode in modes:
+        result = RenderResult(plugin_id, width, height, mode)
+        dm = BoundsCheckingDisplayManager(width=width, height=height, overflow_extent=extent)
+        try:
+            inst = _instantiate(plugin_id, manifest, plugin_dir, config, mock_data, dm)
+            if run_update:
+                try:
+                    inst.update()
+                except _TOLERATED_UPDATE_ERRORS as e:
+                    # Expected when CI / headless dev has no network: record it
+                    # (surfaced in the report) but don't fail the run.
+                    result.update_error = repr(e)
+                    logger.debug("update() connectivity error for %s [%s]: %s", plugin_id, mode, e)
+                except Exception as e:  # noqa: BLE001 — a non-network update() failure is a real bug
+                    # A regression in update() must not pass green just because
+                    # display() survives, so treat it as a failure of this render.
+                    result.error = repr(e)
+                    logger.warning("update() raised a non-connectivity error for %s [%s]: %s",
+                                   plugin_id, mode, e)
+            if result.error is None:
+                _render_mode(inst, mode)
+                result.image = dm.get_image()
+                result.overflow = dm.check_overflow()
+        except Exception as e:  # noqa: BLE001 — a display crash is a real failure
+            result.error = repr(e)
+        results.append(result)
+
+    return results
+
+
+# ---------------------------------------------------------------------------
+# Golden-image comparison
+# ---------------------------------------------------------------------------
+
+def compare_images(rendered: Image.Image, golden: Image.Image,
+                   max_delta: int = 0, max_diff_pixels: int = 0) -> Tuple[bool, int, int]:
+    """Compare two images. Returns (ok, diff_pixel_count, max_per_channel_delta).
+
+    Tolerances default to exact match; bump them only to absorb known platform
+    anti-aliasing noise (requires a pinned Pillow + bundled fonts for stability).
+    """
+    if rendered.size != golden.size:
+        return False, rendered.size[0] * rendered.size[1], 255
+    a = rendered.convert("RGB")
+    b = golden.convert("RGB")
+    diff = ImageChops.difference(a, b)
+    bbox = diff.getbbox()
+    if bbox is None:
+        return True, 0, 0
+    # Count pixels whose largest per-channel delta exceeds the allowed tolerance,
+    # and track the worst delta seen (for reporting).
+    diff_pixels = 0
+    observed_max = 0
+    for px in diff.crop(bbox).getdata():
+        m = max(px) if isinstance(px, tuple) else px
+        if m > observed_max:
+            observed_max = m
+        if m > max_delta:
+            diff_pixels += 1
+    # Pass when the number of out-of-tolerance pixels is within budget.
+    ok = diff_pixels <= max_diff_pixels
+    return ok, diff_pixels, observed_max
+
+
+def golden_path(golden_dir: Path, width: int, height: int, mode: str) -> Path:
+    """Location of a golden image: <golden_dir>/<WxH>/<mode>.png.
+
+    The mode is sanitized to a safe basename so a mode name with '/' or '..'
+    can't read or write outside the golden directory.
+    """
+    return Path(golden_dir) / size_label(width, height) / f"{safe_mode_filename(mode)}.png"
+
+
+def compare_to_goldens(results: List[RenderResult], golden_dir: Path,
+                       max_delta: int = 0, max_diff_pixels: int = 0) -> List[RenderResult]:
+    """Compare rendered results against committed goldens, mutating each result's
+    golden_* fields. Results with no golden file on disk are left unchecked."""
+    for r in results:
+        if r.image is None:
+            continue
+        gp = golden_path(golden_dir, r.width, r.height, r.mode)
+        if not gp.exists():
+            continue
+        r.golden_checked = True
+        with Image.open(gp) as g:
+            ok, diff_pixels, observed_max = compare_images(
+                r.image, g, max_delta=max_delta, max_diff_pixels=max_diff_pixels)
+        r.golden_ok = ok
+        r.golden_diff_pixels = diff_pixels
+        r.golden_max_delta = observed_max
+    return results
+
+
+def write_goldens(results: List[RenderResult], golden_dir: Path) -> int:
+    """Write each successfully-rendered result to its golden path. Returns count."""
+    written = 0
+    for r in results:
+        if r.image is None or r.error is not None:
+            continue
+        gp = golden_path(golden_dir, r.width, r.height, r.mode)
+        gp.parent.mkdir(parents=True, exist_ok=True)
+        r.image.save(gp, format="PNG")
+        written += 1
+    return written

src/plugin_system/testing/loading.py

@@ -0,0 +1,82 @@
+"""
+Shared helpers for loading a plugin headlessly.
+
+Used by scripts/render_plugin.py, scripts/check_plugin.py, and the harness so
+plugin discovery / manifest / config-default logic lives in exactly one place.
+"""
+
+import json
+from pathlib import Path
+from typing import Any, Dict, Optional, Sequence, Union
+
+
+def find_plugin_dir(plugin_id: str, search_dirs: Sequence[Union[str, Path]]) -> Optional[Path]:
+    """Find a plugin directory by searching multiple paths."""
+    from src.plugin_system.plugin_loader import PluginLoader
+    loader = PluginLoader()
+    for search_dir in search_dirs:
+        search_path = Path(search_dir)
+        if not search_path.exists():
+            continue
+        result = loader.find_plugin_directory(plugin_id, search_path)
+        if result:
+            return Path(result)
+    return None
+
+
+def load_manifest(plugin_dir: Union[str, Path]) -> Dict[str, Any]:
+    """Load and return manifest.json from a plugin directory."""
+    manifest_path = Path(plugin_dir) / 'manifest.json'
+    if not manifest_path.exists():
+        raise FileNotFoundError(f"No manifest.json in {plugin_dir}")
+    with open(manifest_path, 'r') as f:
+        return json.load(f)
+
+
+def load_config_defaults(plugin_dir: Union[str, Path]) -> Dict[str, Any]:
+    """Extract default values from a plugin's config_schema.json (empty if none)."""
+    schema_path = Path(plugin_dir) / 'config_schema.json'
+    if not schema_path.exists():
+        return {}
+    with open(schema_path, 'r') as f:
+        schema = json.load(f)
+    defaults: Dict[str, Any] = {}
+    for key, prop in schema.get('properties', {}).items():
+        if isinstance(prop, dict) and 'default' in prop:
+            defaults[key] = prop['default']
+    return defaults
+
+
+def load_harness_spec(plugin_dir: Union[str, Path]) -> Dict[str, Any]:
+    """Optional per-plugin harness settings from <plugin>/test/harness.json.
+
+    Lets a plugin opt into golden-image testing by declaring how to render it
+    deterministically. All keys optional:
+        {
+          "config":     {...},            # config overrides
+          "mock_data":  "fixtures/mock.json",  # path (relative to plugin dir) to cache fixtures
+          "freeze_time": "2025-08-01 15:25:00",
+          "skip_update": false
+        }
+    Returns {} when no harness.json exists.
+    """
+    spec_path = Path(plugin_dir) / 'test' / 'harness.json'
+    if not spec_path.exists():
+        return {}
+    with open(spec_path, 'r') as f:
+        spec = json.load(f)
+
+    # Resolve mock_data path and inline its contents for convenience.
+    mock_rel = spec.get('mock_data')
+    if mock_rel:
+        mock_path = Path(plugin_dir) / mock_rel
+        if not mock_path.exists():
+            # A declared-but-missing fixture is a harness config error: failing
+            # loudly beats silently rendering the plugin with no mock data.
+            raise FileNotFoundError(
+                f"harness.json references mock_data '{mock_rel}' but "
+                f"{mock_path} does not exist"
+            )
+        with open(mock_path, 'r') as mf:
+            spec['mock_data_contents'] = json.load(mf)
+    return spec

src/plugin_system/testing/mocks.py

@@ -63,11 +63,23 @@ class MockCacheManager:
     """Mock cache manager for testing."""
     
     def __init__(self):
+        import shutil
+        import tempfile
+        import weakref
         self._cache: Dict[str, Any] = {}
         self._cache_timestamps: Dict[str, float] = {}
         self.get_calls = []
         self.set_calls = []
         self.delete_calls = []
+        # Real temp dir for plugins that write/read files under cache_dir.
+        # Registered for cleanup so each mock instance doesn't leak a tmp dir.
+        self.cache_dir = tempfile.mkdtemp(prefix="ledmatrix-mock-cache-")
+        self._finalizer = weakref.finalize(
+            self, shutil.rmtree, self.cache_dir, ignore_errors=True)
+
+    def cleanup(self) -> None:
+        """Remove the temp cache directory created for this instance."""
+        self._finalizer()
     
     def get(self, key: str, max_age: Optional[float] = None) -> Optional[Any]:
         """Get a value from cache."""

src/plugin_system/testing/sizes.py

@@ -0,0 +1,120 @@
+"""
+LED matrix sizes the plugin safety harness renders against.
+
+There is no fixed set of "supported" panel sizes — an RGB matrix build can be
+any width/height and configuration (square, rectangle, 2x2, 4x4, 8x2, long
+strips, tall stacks, ...). Plugins are expected to read width/height
+dynamically and lay themselves out accordingly, so the harness's job is to
+prove a plugin survives a *spread* of shapes, not a canonical list.
+
+`DEFAULT_TEST_SIZES` is therefore a representative SAMPLE chosen to span the
+axes of variation (narrow, wide, square, tall, small, long), not an
+exhaustive or authoritative list. Callers can override it entirely:
+
+  - CLI:        scripts/check_plugin.py --sizes 8x16,64x64,256x32
+  - pytest:     LEDMATRIX_TEST_SIZES="8x16,64x64" env var (all plugins), or
+                per-plugin test/harness.json {"sizes": [[8, 16], [64, 64]]}
+
+so anyone can point the harness at the exact panel(s) their build uses.
+"""
+
+import os
+from typing import Iterable, List, Optional, Sequence, Tuple, Union
+
+# A spread of real panel-grid arrangements (each module is 64x32), not a list of
+# "blessed" sizes. Each entry exercises a different layout assumption a plugin
+# might accidentally bake in. Annotations are the panel grid (cols x rows).
+DEFAULT_TEST_SIZES: List[Tuple[int, int]] = [
+    (64, 32),    # 1x1 — single panel, the tightest common rectangle
+    (128, 32),   # 2x1 — the baseline most plugins are tuned for
+    (64, 64),    # 1x2 — stacked, exercises tall-narrow centering
+    (128, 64),   # 2x2 — block, icon scaling / vertical centering
+    (256, 32),   # 4x1 — long strip, wide horizontal layout
+    (128, 96),   # 2x3 — tall, exercises vertical overflow
+    (256, 128),  # 4x4 — large block, both dimensions big at once
+]
+
+# Backwards-compatible alias. Prefer DEFAULT_TEST_SIZES in new code — the old
+# name implied these were the only valid panel sizes, which they are not.
+SUPPORTED_SIZES = DEFAULT_TEST_SIZES
+
+
+def size_label(width: int, height: int) -> str:
+    """Human/path-friendly label for a size, e.g. '128x32'."""
+    return f"{width}x{height}"
+
+
+def parse_size_token(token: str) -> Tuple[int, int]:
+    """Parse a single 'WxH' token into an (int, int) pair.
+
+    Raises ValueError (with a user-friendly message) on malformed input so
+    callers can surface it however they like.
+    """
+    cleaned = token.strip().lower()
+    if "x" not in cleaned:
+        raise ValueError(f"Invalid size '{token}' (expected WxH, e.g. 128x32)")
+    w, h = cleaned.split("x", 1)
+    try:
+        width, height = int(w), int(h)
+    except ValueError as exc:
+        raise ValueError(
+            f"Invalid size '{token}' (expected numeric WxH, e.g. 128x32)"
+        ) from exc
+    if width <= 0 or height <= 0:
+        raise ValueError(
+            f"Invalid size '{token}' (width and height must be positive, e.g. 128x32)"
+        )
+    return (width, height)
+
+
+def coerce_sizes(
+    value: Union[str, Iterable[Sequence[int]], None]
+) -> Optional[List[Tuple[int, int]]]:
+    """Normalize a size spec into a list of (w, h) tuples, or None if empty.
+
+    Accepts a comma-separated 'WxH,WxH' string (CLI / env var) or an iterable
+    of [w, h] / (w, h) pairs (harness.json). Returns None when value is falsy
+    so callers can fall back to the default sample.
+    """
+    if not value:
+        return None
+    if isinstance(value, str):
+        return [parse_size_token(tok) for tok in value.split(",") if tok.strip()]
+    sizes: List[Tuple[int, int]] = []
+    for pair in value:
+        w, h = pair  # raises if not a 2-element sequence
+        width, height = int(w), int(h)
+        if width <= 0 or height <= 0:
+            raise ValueError(f"Invalid size pair {pair!r} (width and height must be positive)")
+        sizes.append((width, height))
+    return sizes or None
+
+
+def resolve_test_sizes(
+    spec_sizes: Union[str, Iterable[Sequence[int]], None] = None,
+) -> List[Tuple[int, int]]:
+    """Decide which sizes to render, by precedence:
+
+    1. LEDMATRIX_TEST_SIZES env var — a global "test on my hardware" override
+       that wins for every plugin.
+    2. spec_sizes — e.g. a per-plugin harness.json "sizes" list.
+    3. DEFAULT_TEST_SIZES — the representative sample.
+    """
+    env = coerce_sizes(os.environ.get("LEDMATRIX_TEST_SIZES"))
+    if env:
+        return env
+    spec = coerce_sizes(spec_sizes)
+    if spec:
+        return spec
+    return list(DEFAULT_TEST_SIZES)
+
+
+def safe_mode_filename(mode: str) -> str:
+    """A filesystem-safe basename for a plugin mode.
+
+    Mode names come from plugin metadata/render state, so a value containing
+    '/' or '..' could otherwise escape the intended output directory. Collapse
+    anything that isn't alphanumeric / dash / underscore to '_'.
+    """
+    cleaned = "".join(ch if ch.isalnum() or ch in ("-", "_") else "_" for ch in mode)
+    return cleaned or "mode"

test/plugins/test_harness.py

@@ -0,0 +1,182 @@
+"""
+Unit tests for the plugin safety harness primitives:
+bounds detection, image comparison, and mode enumeration.
+
+These don't load real plugins, so they run anywhere (including core CI where
+plugin-repos is empty).
+"""
+
+import pytest
+from PIL import Image
+
+from src.plugin_system.testing.bounds_display_manager import BoundsCheckingDisplayManager
+from src.plugin_system.testing.harness import (
+    _TOLERATED_UPDATE_ERRORS, compare_images, list_modes,
+)
+from src.plugin_system.testing.sizes import (
+    DEFAULT_TEST_SIZES, coerce_sizes, parse_size_token, resolve_test_sizes,
+)
+
+
+class TestBoundsDetection:
+    def test_reports_declared_size_not_canvas_size(self):
+        dm = BoundsCheckingDisplayManager(width=64, height=32)
+        assert dm.width == 64 and dm.height == 32
+        assert dm.matrix.width == 64 and dm.matrix.height == 32
+        # Backing canvas is padded out past the declared panel so far-overshoot
+        # coordinates land on-canvas and get flagged instead of clipped.
+        canvas_w, canvas_h = dm.image.size
+        assert canvas_w > 64 and canvas_h > 32
+
+    def test_far_overshoot_on_small_panel_is_detected(self):
+        # A coordinate meant for a wide build (x past 64) must still be caught
+        # when the declared panel is only 64 wide.
+        dm = BoundsCheckingDisplayManager(width=64, height=32)
+        dm.draw.rectangle([200, 5, 210, 10], fill=(255, 0, 0))
+        bbox = dm.check_overflow()
+        assert bbox is not None
+        assert bbox[0] >= 64
+
+    def test_in_bounds_drawing_has_no_overflow(self):
+        dm = BoundsCheckingDisplayManager(width=64, height=32)
+        dm.draw.rectangle([0, 0, 63, 31], fill=(255, 255, 255))
+        assert dm.check_overflow() is None
+
+    def test_right_overflow_is_detected(self):
+        dm = BoundsCheckingDisplayManager(width=64, height=32)
+        # Draw a few pixels past the right edge.
+        dm.draw.rectangle([60, 5, 70, 10], fill=(255, 0, 0))
+        bbox = dm.check_overflow()
+        assert bbox is not None
+        assert bbox[0] >= 64  # overflow starts at or past the declared width
+
+    def test_bottom_overflow_is_detected(self):
+        dm = BoundsCheckingDisplayManager(width=64, height=32)
+        dm.draw.rectangle([5, 30, 10, 40], fill=(0, 255, 0))
+        bbox = dm.check_overflow()
+        assert bbox is not None
+        assert bbox[3] > 32  # overflow extends past the declared height
+
+    def test_declared_image_is_cropped_to_panel(self):
+        dm = BoundsCheckingDisplayManager(width=64, height=32)
+        assert dm.get_image().size == (64, 32)
+
+    def test_snapshot_saves_cropped_panel(self, tmp_path):
+        dm = BoundsCheckingDisplayManager(width=128, height=32)
+        out = tmp_path / "snap.png"
+        dm.save_snapshot(str(out))
+        with Image.open(out) as img:
+            assert img.size == (128, 32)
+
+
+class TestArbitraryPanelSizes:
+    """The harness must handle any panel shape, not a fixed supported list."""
+
+    def test_overflow_extent_pads_to_largest_in_run(self):
+        # A wide run (extent 256) means content at x=200 on a 64-wide panel is
+        # caught; the same draw with a small extent would be clipped (false pass).
+        wide = BoundsCheckingDisplayManager(width=64, height=32, overflow_extent=(256, 32))
+        wide.draw.rectangle([200, 5, 210, 10], fill=(255, 0, 0))
+        assert wide.check_overflow() is not None
+
+        tight = BoundsCheckingDisplayManager(width=64, height=32, overflow_extent=(64, 32))
+        tight.draw.rectangle([200, 5, 210, 10], fill=(255, 0, 0))
+        assert tight.check_overflow() is None  # clipped beyond the small canvas
+
+    def test_unusual_shapes_report_their_declared_size(self):
+        for w, h in [(8, 2), (6, 6), (200, 8), (64, 96)]:
+            dm = BoundsCheckingDisplayManager(width=w, height=h)
+            assert dm.width == w and dm.height == h
+            assert dm.matrix.width == w and dm.matrix.height == h
+
+
+class TestUpdateErrorClassification:
+    """update() may fail for lack of network (tolerated) but a logic bug must
+    not pass green just because display() survives."""
+
+    def test_connectivity_errors_are_tolerated(self):
+        import socket
+        import urllib.error
+        for exc in (ConnectionError("x"), TimeoutError("x"), socket.gaierror("x"),
+                    urllib.error.URLError("x")):
+            assert isinstance(exc, _TOLERATED_UPDATE_ERRORS)
+
+    def test_logic_errors_are_not_tolerated(self):
+        for exc in (ValueError("x"), KeyError("x"), AttributeError("x"), TypeError("x")):
+            assert not isinstance(exc, _TOLERATED_UPDATE_ERRORS)
+
+
+class TestSizeParsing:
+    def test_parse_size_token_ok(self):
+        assert parse_size_token(" 128X32 ") == (128, 32)
+
+    def test_parse_size_token_rejects_garbage(self):
+        with pytest.raises(ValueError):
+            parse_size_token("128xabc")
+        with pytest.raises(ValueError):
+            parse_size_token("128-32")
+
+    def test_rejects_non_positive_dimensions(self):
+        for bad in ("0x32", "-64x32", "64x0", "64x-1"):
+            with pytest.raises(ValueError):
+                parse_size_token(bad)
+        with pytest.raises(ValueError):
+            coerce_sizes([[0, 32]])
+        with pytest.raises(ValueError):
+            coerce_sizes("64x-1")
+
+    def test_coerce_sizes_from_string_and_pairs(self):
+        assert coerce_sizes("8x16,64x64") == [(8, 16), (64, 64)]
+        assert coerce_sizes([[8, 16], (64, 64)]) == [(8, 16), (64, 64)]
+        assert coerce_sizes(None) is None
+        assert coerce_sizes("") is None
+
+    def test_resolve_precedence_env_then_spec_then_default(self, monkeypatch):
+        monkeypatch.delenv("LEDMATRIX_TEST_SIZES", raising=False)
+        assert resolve_test_sizes(None) == list(DEFAULT_TEST_SIZES)
+        assert resolve_test_sizes([[8, 16]]) == [(8, 16)]
+        monkeypatch.setenv("LEDMATRIX_TEST_SIZES", "5x5")
+        # env wins over a per-plugin spec
+        assert resolve_test_sizes([[8, 16]]) == [(5, 5)]
+
+
+class TestCompareImages:
+    def test_identical_images_match(self):
+        a = Image.new("RGB", (16, 16), (10, 20, 30))
+        b = a.copy()
+        ok, diff_pixels, max_delta = compare_images(a, b)
+        assert ok and diff_pixels == 0 and max_delta == 0
+
+    def test_different_images_fail_at_zero_tolerance(self):
+        a = Image.new("RGB", (16, 16), (0, 0, 0))
+        b = a.copy()
+        b.putpixel((1, 1), (255, 255, 255))
+        ok, diff_pixels, max_delta = compare_images(a, b)
+        assert not ok and diff_pixels == 1 and max_delta == 255
+
+    def test_tolerance_absorbs_small_noise(self):
+        a = Image.new("RGB", (16, 16), (100, 100, 100))
+        b = a.copy()
+        b.putpixel((2, 2), (103, 100, 100))  # delta 3
+        ok, _, max_delta = compare_images(a, b, max_delta=5, max_diff_pixels=0)
+        assert ok and max_delta == 3
+
+    def test_size_mismatch_fails(self):
+        a = Image.new("RGB", (16, 16))
+        b = Image.new("RGB", (32, 16))
+        ok, _, _ = compare_images(a, b)
+        assert not ok
+
+
+class TestListModes:
+    def test_instance_modes_take_precedence(self):
+        inst = type("P", (), {"modes": ["a", "b"]})()
+        assert list_modes(inst, {"display_modes": ["x"]}, "pid") == ["a", "b"]
+
+    def test_falls_back_to_manifest_display_modes(self):
+        inst = type("P", (), {})()
+        assert list_modes(inst, {"display_modes": ["x", "y"]}, "pid") == ["x", "y"]
+
+    def test_falls_back_to_plugin_id(self):
+        inst = type("P", (), {})()
+        assert list_modes(inst, {}, "pid") == ["pid"]

test/plugins/test_plugin_matrix.py

@@ -0,0 +1,115 @@
+"""
+Cross-size / cross-screen plugin safety test.
+
+For every discovered plugin, render every declared screen at every supported
+matrix size and assert it: loads, renders without crashing, stays within the
+panel bounds, and — for plugins that ship golden images — matches them.
+
+Plugin discovery (first match wins):
+  - $LEDMATRIX_PLUGINS_DIR  (os.pathsep-separated list of dirs), else
+  - <project_root>/plugin-repos and <project_root>/plugins
+
+A plugin opts into golden-image checks by adding test/golden/<WxH>/<mode>.png
+(and usually test/harness.json for deterministic config / mock data / time).
+"""
+
+import os
+from pathlib import Path
+from typing import Dict, List
+
+import pytest
+
+from src.plugin_system.testing.harness import (
+    render_plugin_matrix, compare_to_goldens,
+)
+from src.plugin_system.testing.loading import load_config_defaults, load_harness_spec
+from src.plugin_system.testing.sizes import resolve_test_sizes
+
+PROJECT_ROOT = Path(__file__).resolve().parents[2]
+
+# Set LEDMATRIX_REQUIRE_PLUGINS=1 in any CI/hardware pipeline where plugins are
+# expected to be present, so a discovery drift (empty search path) fails loudly
+# instead of silently skipping and losing this safety signal.
+_REQUIRE_PLUGINS = os.environ.get("LEDMATRIX_REQUIRE_PLUGINS") == "1"
+
+
+def _plugin_search_dirs() -> List[Path]:
+    env = os.environ.get("LEDMATRIX_PLUGINS_DIR")
+    if env:
+        return [Path(p) for p in env.split(os.pathsep) if p]
+    return [PROJECT_ROOT / "plugin-repos", PROJECT_ROOT / "plugins"]
+
+
+def _discover() -> Dict[str, Path]:
+    """Map plugin_id -> plugin_dir for all plugins on the search path."""
+    found: Dict[str, Path] = {}
+    for base in _plugin_search_dirs():
+        if not base.exists():
+            continue
+        for child in sorted(base.iterdir()):
+            if (child / "manifest.json").exists() and child.name not in found:
+                found[child.name] = child
+    return found
+
+
+_PLUGINS = _discover()
+
+
+@pytest.mark.plugin
+def test_plugins_were_discovered() -> None:
+    """Guard against silently skipping the whole matrix when discovery drifts.
+
+    Local dev and the plugin-less core CI legitimately have no plugins, so we
+    skip there; but when LEDMATRIX_REQUIRE_PLUGINS=1 an empty search path is a
+    hard failure rather than a green no-op.
+    """
+    if _PLUGINS:
+        return
+    search = [str(p) for p in _plugin_search_dirs()]
+    if _REQUIRE_PLUGINS:
+        pytest.fail(
+            "LEDMATRIX_REQUIRE_PLUGINS=1 but no plugins were discovered on the "
+            f"search path: {search}"
+        )
+    pytest.skip(f"no plugins found on the search path: {search}")
+
+
+@pytest.mark.plugin
+@pytest.mark.skipif(not _PLUGINS, reason="no plugins found on the search path")
+@pytest.mark.parametrize("plugin_id", sorted(_PLUGINS))
+def test_plugin_renders_across_sizes_and_screens(plugin_id: str) -> None:
+    plugin_dir = _PLUGINS[plugin_id]
+    spec = load_harness_spec(plugin_dir)
+
+    config = {"enabled": True}
+    config.update(load_config_defaults(plugin_dir))
+    config.update(spec.get("config", {}))
+
+    # Sizes: LEDMATRIX_TEST_SIZES env (test on real hardware) wins, then the
+    # plugin's own harness.json "sizes", else the default representative sample.
+    sizes = resolve_test_sizes(spec.get("sizes"))
+
+    results = render_plugin_matrix(
+        plugin_id=plugin_id,
+        plugin_dir=plugin_dir,
+        config=config,
+        mock_data=spec.get("mock_data_contents", {}),
+        sizes=sizes,
+        run_update=not spec.get("skip_update", False),
+        freeze_time=spec.get("freeze_time"),
+    )
+    compare_to_goldens(results, plugin_dir / "test" / "golden")
+
+    failures = []
+    for r in results:
+        if r.error is not None:
+            failures.append(f"{r.size_label} {r.mode}: crashed: {r.error}")
+        elif r.overflow is not None:
+            failures.append(f"{r.size_label} {r.mode}: overflow past panel bbox={r.overflow}")
+        elif r.golden_checked and r.golden_ok is False:
+            failures.append(
+                f"{r.size_label} {r.mode}: golden drift {r.golden_diff_pixels}px "
+                f"(max Δ={r.golden_max_delta})"
+            )
+
+    assert not failures, f"{plugin_id} failed:\n  " + "\n  ".join(failures)