Made of little things.

// Copy to tests/multiplayer.mjs and adapt the UI selectors and observation below.
// Run with: soyli multiplayer tests/multiplayer.mjs --latency 50 --jitter 15
// This scenario is local trusted test code, not part of the sandboxed napplet.
export default async ({ players, check, measure, diagnostics }) => {
    // Replace these selectors with your app's real host/join and readiness UI.
    await players[0].frame.getByRole('button', { name: 'Host game', exact: true }).click();
    await players[1].frame.getByRole('button', { name: 'Join game', exact: true }).click();
    await Promise.all(
        players.map((player) => player.frame.getByText('Connected', { exact: true }).waitFor()),
    );
    check(
        'every player has a connected peer',
        (await diagnostics()).every((rows) => rows.some((p) => p.state === 'connected')),
    );

    // Use a DOM observation or a read-only app test probe that reflects RENDERED local state.
    // data-local-x below is an example, not a soyLI or NAP property. For a canvas app,
    // expose rendered coordinates during development or sample its rendered pixels.
    // Do not time keydown itself: that would pass even if the world never moved.
    const milliseconds = await players[1].frame.evaluate(async () => {
        const canvas = document.querySelector('canvas');
        if (!canvas || !canvas.hasAttribute('data-local-x'))
            throw new Error('Adapt the rendered-state observation to your app.');
        const before = canvas.getAttribute('data-local-x');
        const start = performance.now();
        canvas.dispatchEvent(new KeyboardEvent('keydown', { code: 'KeyD', bubbles: true }));
        try {
            await new Promise((resolve, reject) => {
                const poll = () => {
                    if (canvas.getAttribute('data-local-x') !== before) resolve();
                    else if (performance.now() - start > 2000)
                        reject(new Error('Local player did not move'));
                    else requestAnimationFrame(poll);
                };
                requestAnimationFrame(poll);
            });
            return performance.now() - start;
        } finally {
            canvas.dispatchEvent(new KeyboardEvent('keyup', { code: 'KeyD', bubbles: true }));
        }
    });
    measure('guest input to visible movement', milliseconds, 50);
    // Extend with aim/shot feedback, interpolation, authoritative convergence, leave/rejoin,
    // and bad peer messages. One movement assertion does not cover the whole game.
};