fix(test-utils): Use epoch-based timestamps in event proxy server

The timestamp filtering used process.hrtime() which returns time relative
to an arbitrary process-local reference point. Since the event proxy
server and Playwright test runner are separate Node.js processes, their
hrtime values were not comparable.

This caused waitForTransaction() to sometimes match transactions from
previous tests (stale events leaking from the buffer), leading to flaky
test failures like the react-router-7-lazy-routes duration assertion.

Fix: Use Date.now() (epoch milliseconds) plus a sub-millisecond component
from performance.now() to get high-resolution timestamps that are
comparable across processes.

Author: BYK

dev-packages/test-utils/src/event-proxy-server.ts

@@ -410,10 +410,23 @@ async function retrieveCallbackServerPort(serverName: string): Promise<string> {
 }
 
 /**
- * We do nanosecond checking because the waitFor* calls and the fetch requests may come very shortly after one another.
+ * Get a high-resolution timestamp that is comparable across processes.
+ *
+ * We use Date.now() as the base (epoch milliseconds) plus a sub-millisecond
+ * component from performance.now() to get microsecond-level precision.
+ *
+ * NOTE: We cannot use process.hrtime() because it returns time relative to an
+ * arbitrary process-local reference point. Since the event proxy server and
+ * Playwright test runner are separate processes, their hrtime values are not
+ * comparable, which was causing stale events from previous tests to leak into
+ * later tests.
  */
 function getNanosecondTimestamp(): number {
-  const NS_PER_SEC = 1e9;
-  const [seconds, nanoseconds] = process.hrtime();
-  return seconds * NS_PER_SEC + nanoseconds;
+  // Date.now() gives us epoch milliseconds (comparable across processes)
+  // performance.now() gives us sub-millisecond precision within this process
+  // We use the fractional part of performance.now() to add microseconds
+  const epochMs = Date.now();
+  const subMs = performance.now() % 1; // Get just the fractional part (0.0 to 0.999...)
+  // Convert to nanoseconds: epoch_ms * 1e6 + sub_ms * 1e6
+  return epochMs * 1e6 + Math.floor(subMs * 1e6);
 }