diff --git a/src/views/Sessions/SignalViewer.tsx b/src/views/Sessions/SignalViewer.tsx
index aaa8207..93d500b 100644
--- a/src/views/Sessions/SignalViewer.tsx
+++ b/src/views/Sessions/SignalViewer.tsx
@@ -906,9 +906,15 @@ export default function SignalViewer() {
   const tryInitRenderer = useCallback(() => {
     const base = canvasRef.current;
     const waveform = waveformCanvasRef.current;
-    // Need the base canvas at minimum; the waveform canvas may be null (then the
-    // hybrid runs Canvas2D-only, which is the same automatic fallback path).
-    if (!base) return;
+    // Construct only once BOTH canvases are mounted. The base chrome canvas and
+    // the waveform canvas mount in the same commit but their ref callbacks fire
+    // in DOM order (base first), so requiring both here prevents constructing the
+    // hybrid with a null waveform canvas — which would pin it to the Canvas2D
+    // fallback for the lifetime of the view (the `rendererRef.current` guard
+    // below blocks reconstruction). The waveform canvas is rendered
+    // unconditionally, so this never deadlocks. If WebGL2 is genuinely
+    // unavailable at runtime, HybridSignalRenderer still falls back internally.
+    if (!base || !waveform) return;
     if (rendererRef.current) return; // already constructed
 
     const renderer = new HybridSignalRenderer(base, waveform, colorResolver);
diff --git a/src/views/Sessions/__tests__/SignalViewer.webglWiring.test.tsx b/src/views/Sessions/__tests__/SignalViewer.webglWiring.test.tsx
new file mode 100644
index 0000000..2627dc3
--- /dev/null
+++ b/src/views/Sessions/__tests__/SignalViewer.webglWiring.test.tsx
@@ -0,0 +1,172 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import { render, waitFor } from '@testing-library/react';
+import { MemoryRouter, Routes, Route } from 'react-router-dom';
+
+import type { SignalManifest } from '@/services/storage/OPFSService';
+
+/**
+ * Regression guard for the WebGL2-activation bug in the Signal Viewer.
+ *
+ * The bug (fixed on this branch): `tryInitRenderer` constructed the
+ * {@link HybridSignalRenderer} as soon as the BASE chrome canvas mounted,
+ * tolerating a null waveform canvas (`if (!base) return;`). Because the base
+ * `<canvas>` precedes the transparent WebGL waveform `<canvas>` in the DOM, the
+ * base ref-callback fires first, so the renderer was ALWAYS constructed as
+ * `new HybridSignalRenderer(base, null, ...)` (waveform null) — pinning it to the
+ * Canvas2D fallback for the lifetime of the view (the `rendererRef.current`
+ * guard blocks reconstruction once the waveform canvas mounts later). Net
+ * effect: WebGL2 never activated in the real Signal Viewer for anyone.
+ *
+ * The fidelity gate constructs the renderers directly via a harness, bypassing
+ * SignalViewer, which is exactly why nothing caught this. This test mounts the
+ * REAL `SignalViewer` and asserts the hybrid renderer is constructed with a
+ * non-null waveform `HTMLCanvasElement` as its second argument — i.e. that the
+ * WebGL layer is actually wired up.
+ *
+ * It is RED against the pre-fix `if (!base) return;` (waveform would be null/
+ * undefined) and GREEN against the fixed `if (!base || !waveform) return;`.
+ */
+
+// ── Capture every HybridSignalRenderer constructor invocation ──────
+//
+// Replace the real (WebGL/Canvas2D-heavy) class with a spy that records its
+// constructor arguments and exposes the methods SignalViewer calls as no-op
+// `vi.fn()`s, so the component renders to completion in jsdom.
+const constructorCalls: unknown[][] = [];
+
+vi.mock('@/components/charts/HybridSignalRenderer', () => {
+  class HybridSignalRendererSpy {
+    constructor(...args: unknown[]) {
+      constructorCalls.push(args);
+    }
+    setOverlayCanvas = vi.fn();
+    resize = vi.fn();
+    render = vi.fn();
+    renderOverlay = vi.fn();
+    renderDuringPan = vi.fn();
+    beginPan = vi.fn();
+    panBy = vi.fn();
+    endPan = vi.fn();
+    getValuesAtTime = vi.fn(() => []);
+    dispose = vi.fn();
+  }
+  return { HybridSignalRenderer: HybridSignalRendererSpy };
+});
+
+// ── Minimal session data so the viewer reaches the canvas JSX ──────
+//
+// OPFSService must report support (else the viewer renders the "Browser Not
+// Supported" branch and never mounts the canvases) and yield a one-channel
+// manifest plus channel data.
+const TEST_SESSION_ID = 'sess-webgl';
+const SESSION_START_MS = Date.parse('2026-01-15T23:00:00Z');
+
+const TEST_MANIFEST: SignalManifest = {
+  version: 1,
+  sessionId: TEST_SESSION_ID,
+  startTime: SESSION_START_MS,
+  endTime: SESSION_START_MS + 60_000,
+  durationSeconds: 60,
+  chunkDurationSeconds: 300,
+  channels: [
+    {
+      index: 0,
+      name: 'flow',
+      sampleRate: 25,
+      unit: 'L/s',
+      dtype: 'float32',
+      physicalMin: -1,
+      physicalMax: 1,
+    },
+  ],
+  chunks: [],
+};
+
+vi.mock('@/services/storage/OPFSService', () => {
+  class OPFSServiceMock {
+    static isSupported = vi.fn(() => true);
+    initialize = vi.fn().mockResolvedValue(undefined);
+    readManifest = vi.fn().mockResolvedValue(TEST_MANIFEST);
+    readChannel = vi.fn().mockResolvedValue(
+      // A few non-zero samples so the channel is not detected as "empty".
+      Float32Array.from([0.1, 0.4, -0.3, 0.2, -0.1]),
+    );
+  }
+  return { OPFSService: OPFSServiceMock };
+});
+
+// Session detail + events come from useSignalData; return a ready session with
+// no events so the viewer is fully hydrated without touching IndexedDB.
+vi.mock('@/hooks/useSignalData', () => ({
+  useSessionDetail: () => ({
+    session: {
+      id: TEST_SESSION_ID,
+      startTime: new Date(SESSION_START_MS).toISOString(),
+      date: '2026-01-15',
+    },
+    aggregate: null,
+    loading: false,
+    error: null,
+  }),
+  useEventData: () => ({ events: [], loading: false, error: null }),
+}));
+
+// No wearable lanes and no breathing detection — keeps the render minimal and
+// independent of those subsystems.
+// Partial mock: keep the module's real exports (signalLanes.ts imports
+// SLEEP_STAGE_CODES from here) and override only the hook.
+vi.mock('@/hooks/useWearableLanes', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('@/hooks/useWearableLanes')>();
+  return {
+    ...actual,
+    useWearableLanes: () => ({ series: {}, loading: false, error: null }),
+  };
+});
+
+vi.mock('@/hooks/useBreathingEpisodes', () => ({
+  useBreathingEpisodes: () => ({ episodes: [], loading: false, error: null }),
+}));
+
+// Import AFTER the mocks are registered.
+import SignalViewer from '../SignalViewer';
+
+function renderViewer() {
+  return render(
+    <MemoryRouter initialEntries={[`/sessions/${TEST_SESSION_ID}`]}>
+      <Routes>
+        <Route path="/sessions/:sessionId" element={<SignalViewer />} />
+      </Routes>
+    </MemoryRouter>,
+  );
+}
+
+describe('SignalViewer WebGL wiring', () => {
+  beforeEach(() => {
+    constructorCalls.length = 0;
+    localStorage.clear();
+  });
+
+  it('constructs HybridSignalRenderer with a non-null waveform canvas (WebGL layer wired)', async () => {
+    renderViewer();
+
+    // The renderer is constructed by a canvas ref-callback once both the base
+    // and waveform canvases have mounted (after the async OPFS manifest load).
+    await waitFor(() => {
+      expect(constructorCalls.length).toBeGreaterThan(0);
+    });
+
+    const [baseArg, waveformArg] = constructorCalls[0] as [unknown, unknown, unknown];
+
+    // Pre-fix, the renderer was built the moment the BASE canvas mounted, with a
+    // still-null waveform canvas. The whole point of the fix is that BOTH canvases
+    // exist at construction time — so the second argument must be a real canvas.
+    expect(waveformArg).not.toBeNull();
+    expect(waveformArg).not.toBeUndefined();
+    expect(waveformArg).toBeInstanceOf(HTMLCanvasElement);
+
+    // The base canvas (first argument) must also be a real canvas, and the two
+    // must be distinct elements (base chrome layer vs. transparent WebGL layer).
+    expect(baseArg).toBeInstanceOf(HTMLCanvasElement);
+    expect(waveformArg).not.toBe(baseArg);
+  });
+});