Async mode takes at least 4ms per iteration in browsers because of setTimeout clamping #709

In async mode, each iteration of the diff loop runs in a setTimeout(fn, 0) that the previous iteration scheduled. Browsers clamp nested timers to at least 4ms once they nest more than 5 deep (HTML spec), and Node turns a 0ms delay into 1ms. So in a browser an async diff takes at least 4ms per iteration, no matter how little work each iteration does.

diffWords on generated text in Chrome 154:

Node 24 shows the same thing with a 1ms floor: 193 ms sync vs 2177 ms async on the last input.

That makes async mode impractical for its main job, which is keeping a page responsive while diffing a large document. It also uses up timeout, which counts the time spent waiting on timers.

Suggested fix: yield through a MessageChannel instead. Posted messages aren't clamped, and MessageChannel is both Baseline widely available and a global in current Node LTS, so it fits the compatibility policy. React's scheduler yields this way for the same reason. The same inputs with that change:

Chrome painted 15 frames during the slowest one, so the page stayed responsive. Each port has to be closed after its message, or it keeps a Node process alive.

I'll open a PR with this change.