test(conversation): run the momentum-cap test on a set clock

The test slept in real time and read the wall clock, so on a loaded
machine a gap between two scrolls could outlast the gesture's quiet
window and end the reader's gesture before the cap. It now moves
performance.now itself, 100ms per scroll, and checks every scroll on
each side of the two-second cap.

Author: pratikbuilds

src/ui/conversation.test.tsx

@@ -643,32 +643,41 @@ describe("reader attribution and scroll listeners", () => {
   });
 
   test("scrolls after the momentum cap are not the reader's", async () => {
-    const { mounted, element, current } = mountPolicy("stick");
-    giveGeometry(element, 10_000, 500);
-    element.scrollTop = 9_500;
-    const seen: { at: number; byReader: boolean }[] = [];
-    current().subscribeScroll((snapshot) =>
-      seen.push({ at: performance.now(), byReader: snapshot.byReader }),
-    );
-    await nextFrame();
-
-    flick(element, 9_000);
-    const lifted = performance.now();
-    let top = 9_000;
-    while (performance.now() - lifted < 2_600) {
-      await sleep(100);
-      top -= 10;
-      element.scrollTop = top;
-      element.dispatchEvent(new Event("scroll"));
+    // A clock the test moves, so a slow machine can't stretch the gap
+    // between two scrolls past the gesture's quiet window.
+    let now = 0;
+    const originalNow = performance.now.bind(performance);
+    performance.now = () => now;
+    try {
+      const { mounted, element, current } = mountPolicy("stick");
+      giveGeometry(element, 10_000, 500);
+      element.scrollTop = 9_500;
+      const seen: { at: number; byReader: boolean }[] = [];
+      current().subscribeScroll((snapshot) =>
+        seen.push({ at: now, byReader: snapshot.byReader }),
+      );
       await nextFrame();
+
+      flick(element, 9_000);
+      const lifted = now;
+      let top = 9_000;
+      while (now - lifted < 2_600) {
+        now += 100;
+        top -= 10;
+        element.scrollTop = top;
+        element.dispatchEvent(new Event("scroll"));
+        await nextFrame();
+      }
+      const early = seen.filter(({ at }) => at > lifted && at - lifted < 2_000);
+      const late = seen.filter(({ at }) => at - lifted > 2_000);
+      expect(early).toHaveLength(19);
+      expect(early.every(({ byReader }) => byReader)).toBe(true);
+      expect(late).toHaveLength(6);
+      expect(late.every(({ byReader }) => !byReader)).toBe(true);
+      mounted.unmount();
+    } finally {
+      performance.now = originalNow;
     }
-    const early = seen.filter(({ at }) => at > lifted && at - lifted < 1_800);
-    const late = seen.filter(({ at }) => at - lifted > 2_200);
-    expect(early.length).toBeGreaterThan(10);
-    expect(early.every(({ byReader }) => byReader)).toBe(true);
-    expect(late.length).toBeGreaterThan(0);
-    expect(late.every(({ byReader }) => !byReader)).toBe(true);
-    mounted.unmount();
   });
 
   test("a glide started inside the reader's window is not the reader's", async () => {