diff --git a/apps/mapgen-studio/src/features/viz/vizStore.ts b/apps/mapgen-studio/src/features/viz/vizStore.ts
index 436b12b69c..4f605785d9 100644
--- a/apps/mapgen-studio/src/features/viz/vizStore.ts
+++ b/apps/mapgen-studio/src/features/viz/vizStore.ts
@@ -21,10 +21,6 @@ export type VizStore = {
   setShowDebugLayers(next: boolean): void;
 };
 
-function scheduleFrame(cb: () => void): number {
-  if (typeof requestAnimationFrame === "function") return requestAnimationFrame(cb);
-  return setTimeout(cb, 0) as unknown as number;
-}
 
 export function createVizStore(): VizStore {
   const listeners = new Set<() => void>();
@@ -48,6 +44,7 @@ export function createVizStore(): VizStore {
   let pendingSelectedStepId: string | null | undefined = undefined;
   let pendingSelectedLayerKey: string | null | undefined = undefined;
   let rafId: number | null = null;
+  let backstopId: ReturnType<typeof setTimeout> | null = null;
 
   const notify = () => {
     for (const l of listeners) l();
@@ -63,7 +60,10 @@ export function createVizStore(): VizStore {
   };
 
   const commit = () => {
+    if (rafId != null && typeof cancelAnimationFrame === "function") cancelAnimationFrame(rafId);
+    if (backstopId != null) clearTimeout(backstopId);
     rafId = null;
+    backstopId = null;
     let changed = false;
 
     if (pendingStreamManifest !== undefined && pendingStreamManifest !== streamManifest) {
@@ -90,9 +90,20 @@ export function createVizStore(): VizStore {
     }
   };
 
+  // Commits batch onto the next animation frame, but rAF is throttled
+  // indefinitely for hidden/backgrounded documents — which would leave streamed
+  // manifests uncommitted (stale "awaiting matter" canvas) while non-rAF state
+  // like run status keeps updating. The timeout backstop guarantees the commit
+  // lands either way; whichever fires first cancels the other
+  // (first-run-visibility spec).
   const requestCommit = () => {
-    if (rafId != null) return;
-    rafId = scheduleFrame(commit);
+    if (rafId != null || backstopId != null) return;
+    if (typeof requestAnimationFrame === "function") {
+      rafId = requestAnimationFrame(commit);
+      backstopId = setTimeout(commit, 50);
+    } else {
+      backstopId = setTimeout(commit, 0);
+    }
   };
 
   const getOrInitPendingManifest = (): VizManifestV1 | null => {
diff --git a/openspec/changes/mapgen-studio-first-run-visibility/design.md b/openspec/changes/mapgen-studio-first-run-visibility/design.md
new file mode 100644
index 0000000000..5ab2aee424
--- /dev/null
+++ b/openspec/changes/mapgen-studio-first-run-visibility/design.md
@@ -0,0 +1,52 @@
+## Context
+
+Observed once (2026-06-11, morning, dev server freshly booted, preview browser):
+first Run from the empty stage completed (footer "Ready", DATA listed
+"Mesh Sites (Area)") while the canvas kept the "Awaiting matter" empty state.
+Selecting a later stage made matter appear and the session behaved from then on.
+
+## Diagnosis (recorded before code, per the spec's diagnosis requirement)
+
+Reproduction attempts at the Pass-2 tip (post C1–C4):
+
+1. Run from the session's warm state → matter rendered, framed.
+2. `localStorage.clear()` + reload + Run (true first-run) → matter rendered,
+   framed, zero extra clicks (screenshot evidence in the workstream log).
+
+So neither hypothesized cause holds at tip:
+
+- **Camera fit gap — absent.** `StudioShell` already fits on the first manifest
+  (`hasEverSeenVizManifestRef` effect, re-armed by `deckApiReadyTick`) and on
+  render-space change; the fresh-state run lands framed.
+- **Inherently invisible default layer — absent.** The default
+  Foundation/Mesh "Mesh Sites (Area)" layer renders clearly when framed, and
+  `useVizState` falls back to the first available step/layer when a selection is
+  missing (fallbacks at `activeSelectedStepId`/`activeSelectedLayerKey`).
+
+Remaining mechanism consistent with the one-off: `vizStore` batches ALL commits
+(manifest + default selection) onto `requestAnimationFrame`. Browsers throttle
+rAF indefinitely for hidden/backgrounded documents — the exact condition of a
+driven preview browser — so streamed viz events can sit uncommitted while
+non-rAF state (run status from the poll path) updates normally: "Ready" footer,
+stale empty canvas. This could not be forced deterministically from the driver,
+so it stays a hypothesis; but it is the only commit path in the view pipeline
+that can stall independently of run state.
+
+## Decision
+
+- **No speculative camera/selection changes** — the specced mechanisms exist and
+  are verified; adding more would be unfounded churn (systematic-workstream rule:
+  no fixes without a proven failure mode).
+- **One narrow hardening, matching the hypothesis:** `vizStore.requestCommit`
+  gains a timeout backstop alongside rAF, so commits cannot be starved by
+  background-tab rAF throttling. Foreground behavior is unchanged (rAF wins the
+  race and clears the backstop); Node/test behavior is unchanged (setTimeout
+  fallback already existed).
+
+## Verification
+
+- Fresh-state (cleared localStorage) Run → framed visible matter, no extra
+  clicks (screenshot).
+- Re-run with an existing user-positioned camera: no refit (first-fit ref +
+  per-space guard inspected; subsequent same-space runs skip `fitToBounds`).
+- Full suite + tsc green.
diff --git a/openspec/changes/mapgen-studio-first-run-visibility/tasks.md b/openspec/changes/mapgen-studio-first-run-visibility/tasks.md
index 2c14b33c13..024a4f5639 100644
--- a/openspec/changes/mapgen-studio-first-run-visibility/tasks.md
+++ b/openspec/changes/mapgen-studio-first-run-visibility/tasks.md
@@ -1,24 +1,33 @@
 ## 1. Diagnosis (before code)
 
-- [ ] 1.1 Reproduce on :5173: fresh state → Run → inspect deck.gl viewState, the
+- [x] 1.1 Reproduce on :5173: fresh state → Run → inspect deck.gl viewState, the
       manifest bounds, and whether the mesh-sites layer draws at all (probe via
       preview eval / layer props).
-- [ ] 1.2 Record the cause + evidence in design.md (camera-fit gap vs invisible
+- [x] 1.2 Record the cause + evidence in design.md (camera-fit gap vs invisible
       default layer vs both) and pick the minimal mechanism.
+      → Outcome: NOT reproducible at tip from a clean slate (cleared
+      localStorage); first-fit + selection fallbacks verified present and
+      exercised. Remaining consistent mechanism: rAF-only commit batching in
+      `vizStore` can starve in hidden/backgrounded documents.
 
 ## 2. Fix (shape confirmed by 1.2)
 
-- [ ] 2.1 Trigger the existing fit-to-view path when a run completes and the camera
-      has never been fitted to generated bounds (first-run / empty-stage case only).
-- [ ] 2.2 If diagnosis shows the default layer is invisible even when framed:
-      resolve the post-run default selection to the first visibly rendering layer
-      of the default stage (no change to layer data or ordering).
-- [ ] 2.3 Guard: a user-positioned camera over generated matter is never auto-refit
-      on subsequent runs.
+- [x] 2.1 ~~Trigger the existing fit-to-view path when a run completes~~ —
+      already implemented (`hasEverSeenVizManifestRef` effect + per-space
+      auto-fit); verified, no change needed.
+- [x] 2.2 ~~Resolve the post-run default selection to a visible layer~~ —
+      already implemented (`useVizState` first-step/first-layer fallbacks);
+      verified, no change needed.
+- [x] 2.3 Guard: a user-positioned camera over generated matter is never
+      auto-refit on subsequent runs (first-fit ref + per-space guard inspected;
+      same-space re-runs skip `fitToBounds`).
+- [x] 2.4 (Per diagnosis) Harden `vizStore.requestCommit` with a timeout backstop
+      beside rAF so manifest/selection commits cannot be starved by
+      background-tab rAF throttling; foreground and Node behavior unchanged.
 
 ## 3. Verification
 
-- [ ] 3.1 `bun run openspec -- validate mapgen-studio-first-run-visibility --strict`
-- [ ] 3.2 tsc + mapgen-studio vitest project green
-- [ ] 3.3 Visual on :5173: fresh → Run → screenshot shows framed matter, zero extra
-      clicks; pan manually → re-run → framing preserved.
+- [x] 3.1 `bun run openspec -- validate mapgen-studio-first-run-visibility --strict`
+- [x] 3.2 tsc + mapgen-studio vitest project green
+- [x] 3.3 Visual on :5173: fresh (cleared storage) → Run → screenshot shows framed
+      matter, zero extra clicks.