fix(producer): keep page timers on the film's schedule at every worker count

packages/producer/src/services/fileServer-virtualTimers.test.ts

@@ -89,6 +89,16 @@ describe("virtual-time shim timers", () => {
     expect(run("fired")).toBe(11);
   });
 
+  it("keeps the film's timer schedule on a worker whose first frame is past the start", () => {
+    const { run } = page();
+    run(`window.at = [];
+      requestAnimationFrame(() => (function step() { setTimeout(() => { at.push(Date.now()); step(); }, 100); })());`);
+    run("window.__HF_VIRTUAL_TIME__.seekToTime(4000 / 30);");
+    expect(run("at")).toEqual([100]);
+    run("for (let f = 5; f <= 30; f++) window.__HF_VIRTUAL_TIME__.seekToTime(f * 1000 / 30);");
+    expect(run("at")).toEqual([100, 200, 300, 400, 500, 600, 700, 800, 900, 1000]);
+  });
+
   it("runs due timers before the frame's animation callbacks", () => {
     const { run } = page();
     run("window.__HF_VIRTUAL_TIME__.seekToTime(0);");

packages/producer/src/services/fileServer.ts

@@ -293,7 +293,7 @@ export function buildVirtualTimeShim(options: VirtualTimeShimOptions): string {
   // The seekToTime hook reseeds when seeding is on; under seedRandomFromFrame=false
   // we emit no extra call so the function body is byte-identical to the
   // unseeded shim.
-  const seekToTimeReseedCall = seedRandomFromFrame ? "reseedRngFromTime(safeTimeMs);\n      " : "";
+  const seekToTimeReseedCall = seedRandomFromFrame ? "reseedRngFromTime(safeTimeMs);\n    " : "";
   return String.raw`(function() {
   if (window.__HF_VIRTUAL_TIME__) return;
 
@@ -434,21 +434,28 @@ ${seededRandomBlock}
     if (!cancelTimer(id)) originalClearInterval(id);
   };
 
+  function seekToTime(nextTimeMs) {
+    var safeTimeMs = Math.max(0, Number(nextTimeMs) || 0);
+    if (!rendering && window.__renderReady === true) {
+      rendering = true;
+      // A worker whose first frame is past the start plays the film's first frame first, so timers the
+      // page starts there keep the schedule a one-worker render gives them.
+      if (safeTimeMs > 0) seekToTime(0);
+    }
+    ${seekToTimeReseedCall}fireDueTimers(safeTimeMs);
+    virtualNowMs = safeTimeMs;
+    flushAnimationFrame();
+    return virtualNowMs;
+  }
+
   window.__HF_VIRTUAL_TIME__ = {
     originalSetTimeout: originalSetTimeout,
     originalClearTimeout: originalClearTimeout,
     originalSetInterval: originalSetInterval,
     originalClearInterval: originalClearInterval,
     originalRequestAnimationFrame: originalRequestAnimationFrame,
     originalCancelAnimationFrame: originalCancelAnimationFrame,
-    seekToTime: function(nextTimeMs) {
-      var safeTimeMs = Math.max(0, Number(nextTimeMs) || 0);
-      if (window.__renderReady === true) rendering = true;
-      ${seekToTimeReseedCall}fireDueTimers(safeTimeMs);
-      virtualNowMs = safeTimeMs;
-      flushAnimationFrame();
-      return virtualNowMs;
-    },
+    seekToTime: seekToTime,
     getTime: function() {
       return virtualNowMs;
     },