Make browser StopwatchTests robust to coarse clock resolution

[Copilot]

Fixes #62021

StopwatchTests.GetTimestamp and StartNewAndReset fail intermittently on the browser/WASM target: consecutive GetTimestamp() reads return equal values and Elapsed measures zero.

Root cause

On browser, Stopwatch.GetTimestamp() → minipal_hires_ticks() → emscripten clock_gettime(CLOCK_MONOTONIC), backed by performance.now(), which is security-clamped to a coarse resolution. Blocking waits (ManualResetEvent.WaitOne(timeout)) also don't advance real time on the single browser thread. A fixed-timeout sleep therefore isn't guaranteed to move the timestamp past a resolution boundary. The monotonic hi-res clock in use is already the best clock the browser exposes — there is no higher-resolution alternative to switch to — so the test must tolerate coarse resolution rather than assume a fixed sleep crosses a boundary.

Changes

• Stopwatch.cs test Sleep helper: on browser only, spin (re-issuing the existing WaitOne) until the Stopwatch timestamp advances by at least one TimeSpan tick. Non-browser platforms are unchanged.

private static void Sleep(int milliseconds)
{
    if (PlatformDetection.IsBrowser)
    {
        long start = Stopwatch.GetTimestamp();
        long minTicks = Math.Max(1, Stopwatch.Frequency / TimeSpan.TicksPerSecond);
        do
        {
            s_sleepEvent.WaitOne(milliseconds);
        }
        while (Stopwatch.GetTimestamp() - start < minTicks);

        return;
    }

    s_sleepEvent.WaitOne(milliseconds);
}

This guarantees a measurable delta before dependent asserts (Assert.NotEqual(ts1, ts2), Elapsed > TimeSpan.Zero), covering GetTimestamp, StartNewAndReset, ConstructStartAndStop, and OverridesToString. No tests are disabled.

Note

This PR was generated with the assistance of GitHub Copilot.

[Copilot]

Pull request overview

This PR adjusts System.Diagnostics.Tests.StopwatchTests to reduce intermittent failures on browser/WASM by making the test’s sleep helper wait until Stopwatch.GetTimestamp() observably advances, rather than assuming a single fixed wait crosses the clock’s resolution boundary.

Changes:

• Add a browser-specific loop in the Sleep(int milliseconds) helper to retry waiting until the stopwatch timestamp advances.

[pavelsavara]

/ba-g CI failure is #130618