feat(dev): preview a plugin's Vegas strip in the dev server

The dev server's View selector gains "Vegas strip (live elements)" and
"Vegas strip (plain Vegas content)": the plugin's block of the Vegas ticker,
laid out by the ticker's own code (render_vegas_strip, as render_plugin.py
--vegas uses), with its live elements listed. /api/render takes
"vegas": "live" | "plain"; the display view is unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/dev_server.py

@@ -200,12 +200,21 @@ def api_plugin_defaults(plugin_id):
     return jsonify({'defaults': defaults})
 
 
+#: /api/render "vegas" values: the plugin's block of the Vegas strip, built
+#: from its live elements (falling back to its Vegas content, as the ticker
+#: does) or from its ordinary Vegas content only.
+VEGAS_VIEWS = ('live', 'plain')
+
+
 def _render_once(plugin_id, plugin_dir, manifest, config, mock_data, width, height,
-                 skip_update):
+                 skip_update, vegas=None):
     """Render one plugin at one size. Returns the /api/render response dict.
 
     A fresh plugin instance per call, mirroring the safety harness, so sizes
-    never share state.
+    never share state. With ``vegas`` set ('live' or 'plain') the image is the
+    plugin's block of the Vegas strip instead of its display(), laid out by
+    the ticker's own code (src/plugin_system/testing/vegas.py), and the
+    response lists where each live element sits in it.
     """
     from src.plugin_system.testing import VisualTestDisplayManager, MockCacheManager, MockPluginManager
     from src.plugin_system.plugin_loader import PluginLoader
@@ -243,6 +252,10 @@ def _render_once(plugin_id, plugin_dir, manifest, config, mock_data, width, heig
             logger.warning("update() raised for plugin %s", plugin_id, exc_info=True)
             warnings.append(f"update() raised: {type(e).__name__} — see server log")
 
+    if vegas:
+        return _vegas_response(plugin_id, plugin_instance, display_manager, vegas,
+                               start_time, errors, warnings)
+
     # Run display()
     try:
         plugin_instance.display(force_clear=True)
@@ -262,6 +275,40 @@ def _render_once(plugin_id, plugin_dir, manifest, config, mock_data, width, heig
     }
 
 
+def _vegas_response(plugin_id, plugin_instance, display_manager, vegas, start_time,
+                    errors, warnings):
+    """The /api/render response for the Vegas strip view."""
+    import base64
+    import io
+
+    from src.plugin_system.testing.vegas import render_vegas_strip
+
+    block, layout = None, []
+    try:
+        block, layout = render_vegas_strip(plugin_instance, plugin_id, display_manager,
+                                           live=(vegas == 'live'))
+    except Exception as e:
+        logger.warning("Vegas render raised for plugin %s", plugin_id, exc_info=True)
+        errors.append(f"Vegas render raised: {type(e).__name__} — see server log")
+    if block is None:
+        if not errors:
+            errors.append("The plugin has no Vegas content")
+        block = display_manager.image
+    elif vegas == 'live' and not layout:
+        warnings.append("No live elements: this is the plugin's ordinary Vegas content")
+    buffer = io.BytesIO()
+    block.convert('RGB').save(buffer, format='PNG')
+    return {
+        'image': 'data:image/png;base64,' + base64.b64encode(buffer.getvalue()).decode('ascii'),
+        'width': block.width,
+        'height': block.height,
+        'render_time_ms': round((time.time() - start_time) * 1000, 1),
+        'errors': errors,
+        'warnings': warnings,
+        'live_elements': [{'key': key, 'x': x, 'width': width} for x, key, width in layout],
+    }
+
+
 def _trusted_plugin_dir(plugin_dir: Path) -> Optional[Path]:
     """Re-derive a plugin directory from the search dirs' own listings.
 
@@ -333,6 +380,10 @@ def api_render():
     if not (MIN_HEIGHT <= height <= MAX_HEIGHT):
         return jsonify({'error': f'height must be between {MIN_HEIGHT} and {MAX_HEIGHT}'}), 400
 
+    vegas = data.get('vegas') or None
+    if vegas is not None and vegas not in VEGAS_VIEWS:
+        return jsonify({'error': f'vegas must be one of {", ".join(VEGAS_VIEWS)}'}), 400
+
     try:
         plugin_dir, manifest, config, mock_data, skip_update = _parse_render_request(data)
     except LookupError:
@@ -345,7 +396,7 @@ def api_render():
 
     try:
         result = _render_once(data['plugin_id'], plugin_dir, manifest, config,
-                              mock_data, width, height, skip_update)
+                              mock_data, width, height, skip_update, vegas=vegas)
     except Exception:
         app.logger.exception('plugin load failed during render')
         return jsonify({'error': 'Failed to load plugin; see server log'}), 500

scripts/templates/dev_preview.html

@@ -247,6 +247,19 @@
                 </div>
             </details>
 
+            <!-- What to render: the plugin's screen, or its block of the Vegas strip -->
+            <div class="flex items-center gap-2">
+                <label for="viewSelect" class="text-xs whitespace-nowrap" style="color: var(--text-secondary);">View</label>
+                <select id="viewSelect" onchange="onConfigChange()"
+                        class="flex-1 px-2 py-1.5 rounded-lg text-xs"
+                        style="background: var(--bg-primary); color: var(--text-primary); border: 1px solid var(--border-color);"
+                        title="Vegas strip: the plugin's block of the Vegas ticker, laid out as the ticker lays it out">
+                    <option value="">Display</option>
+                    <option value="live">Vegas strip (live elements)</option>
+                    <option value="plain">Vegas strip (plain Vegas content)</option>
+                </select>
+            </div>
+
             <!-- Render buttons -->
             <div class="flex gap-2">
                 <button onclick="renderPlugin()" id="renderBtn"
@@ -489,6 +502,7 @@
                         width: width,
                         height: height,
                         mock_data: mockData,
+                        vegas: document.getElementById('viewSelect').value || null,
                     }),
                 });
 
@@ -510,8 +524,11 @@
                 updateZoom();
 
                 // Show render time
+                const live = data.live_elements;
                 document.getElementById('renderTimeText').textContent =
-                    `${data.render_time_ms}ms`;
+                    `${data.render_time_ms}ms` + (live ? ` · ${data.width}px strip, ` +
+                    `${live.length} live element(s)` +
+                    (live.length ? `: ${live.map(e => e.key).join(', ')}` : '') : '');
 
                 // Show warnings/errors
                 showMessages(data.errors || [], data.warnings || []);

test/test_dev_server_vegas.py

@@ -0,0 +1,49 @@
+"""scripts/dev_server.py's Vegas strip view, on the stub fixture plugin."""
+import importlib.util
+from pathlib import Path
+
+import pytest
+
+REPO = Path(__file__).resolve().parent.parent
+STUB = REPO / "test" / "fixtures" / "plugins" / "vegas-live-stub"
+
+
+@pytest.fixture
+def client(monkeypatch):
+    spec = importlib.util.spec_from_file_location("dev_server_vegas_under_test",
+                                                  REPO / "scripts" / "dev_server.py")
+    dev_server = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(dev_server)
+    monkeypatch.setattr(dev_server, "find_plugin_dir", lambda pid: STUB)
+    monkeypatch.setattr(dev_server, "_trusted_plugin_dir", lambda d: STUB)
+    return dev_server.app.test_client()
+
+
+def _render(client, **extra):
+    return client.post("/api/render", json={"plugin_id": "vegas-live-stub", "width": 192,
+                                            "height": 48, "config": {"enabled": True},
+                                            **extra})
+
+
+def test_the_live_strip_lists_its_elements(client):
+    data = _render(client, vegas="live").get_json()
+    assert data["image"].startswith("data:image/png;base64,")
+    assert data["height"] == 48 and data["width"] > 192
+    keys = [e["key"] for e in data["live_elements"]]
+    assert keys[:2] == ["card:0", "card:1"] and "map" in keys
+    assert not data["errors"]
+
+
+def test_the_plain_strip_has_no_live_elements(client):
+    data = _render(client, vegas="plain").get_json()
+    assert data["live_elements"] == [] and not data["errors"]
+
+
+def test_the_display_view_is_unchanged(client):
+    data = _render(client).get_json()
+    assert (data["width"], data["height"]) == (192, 48)
+    assert "live_elements" not in data
+
+
+def test_an_unknown_view_is_refused(client):
+    assert _render(client, vegas="sideways").status_code == 400