[Epic-Ideas-from-Alex] Threading-model rewrite: one dispatcher, cancellation everywhere, deterministic dispose #3446

🚀 Feature Request

Replace the ad-hoc mix of event handlers, TaskCompletionSources, SemaphoreSlim queues and sync-over-async with a single explicit model:

• One dispatcher that owns all client-side state. Builds on PR fix(threading): guard client-side state with a dispatch lock #3387 (dispatch lock) and the threading notes in PR docs(claude): describe the threading model and client architecture #3385.
• Subscribe-then-check everywhere, removing the race family behind [Bug]: WaitForURLAsync can lose the event it is waiting for — its check-then-subscribe races with the thread that delivers events #3364, [Bug]: WaitForURLAsync(WaitUntil = Load) intermittently times out although load has already fired (readyState === 'complete') #3399, PR fix(frame): subscribe before checking state in navigation waits #3384, PR fix(frame): don't miss a load state fired while the wait is being set up #3400 and PR fix(frame): do not lose a navigation or load state that lands while a wait subscribes #3405.
• CancellationToken on every async API ([Feature]: IAPIRequestContext.GetAsync should be cancellable #3218), wired into the waiters and protocol calls.
• Deterministic shutdown: closing the connection fails every pending and queued call with TargetClosedException. No hangs ([Bug]: Playwright.Dispose() can hang forever while the driver is still sending messages #3419 and the TaskQueue issue), no ObjectDisposedException.
• No sync-over-async anywhere in the library (Locator.ToString, the adapters' ProcessExit).
• A concurrency test suite (parallel WhenAll calls, connection kill mid-flight, events delivered during subscription) run under stress in CI.

Example

using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
await page.GetByRole(AriaRole.Button, new() { Name = "Pay" }).ClickAsync(cancellationToken: cts.Token);

await foreach (var req in page.RequestsAsync(cts.Token)) { /* no lost events */ }

Motivation

Most recent correctness bugs in this repo are races or shutdown hangs, fixed one at a time. They are also the hardest to reproduce, and the most damaging in large parallel suites and on the Playwright Service.

A deliberate model, documented and stress-tested, removes the whole class instead of patching symptoms. It's also the foundation the other .NET-idiomatic features need (IAsyncEnumerable events, cancellation).