Looks like the tests need updated to support threading. On it

Sorry about the one file per commit "commits" right now. Work VPN doesn't want to let me log in via bash or gh desktop

All comments addressed

The revised implementation looks great to me 👍.

Now to verification.
I observe that if I remove the [ThreadStatic] from the current implementation the tests fail, whereas this PR succeeds.
So this PR seems at least as good as master.

I've tried to come up with a test to verify that the changes are also an improvement over master.
The test below fails in master, but succeeds with this PR.
If this test like this gets added (with better names than First and Second) I'm confident with these changes.

[Fact]
public async Task When_using_AssertionScope_across_thread_boundaries_it_should_work()
{
    using (var semaphore = new SemaphoreSlim(0, 1))
    {
        await Task.WhenAll(First(semaphore), Second(semaphore));
    }
}

private static async Task First(SemaphoreSlim semaphore)
{
    await Task.Yield();
    var scope = new AssertionScope();
    await semaphore.WaitAsync();
    scope.Should().BeSameAs(AssertionScope.Current);
}

private static async Task Second(SemaphoreSlim semaphore)
{
    await Task.Yield();
    var scope = new AssertionScope();
    semaphore.Release();
    scope.Should().BeSameAs(AssertionScope.Current);
}

Added Tests

Is there anything else you need in order to get this merged?