Web: queue EventLoopProxy::send_event() to microtask when triggered from outside the event loop

[daxpedda]

This also checks if its coming from a thread or not, in which case it doesn't queue it.

I also noticed some legacy code that deferred to ControlFlow::Poll when receiving events instead of waking up the event loop immediately, which I removed, so now events should come in faster regardless of PollStrategy.

Fixes #3715.

[madsmtm]

Don't understand what's happening here, just gonna leave an approving review so that you can merge it since it only touches the web backend.

A few questions though: Why are we not using wasm_bindgen_futures internally ourselves? And why does web_sys not expose queueMicrotask?

[daxpedda]

A few questions though: Why are we not using wasm_bindgen_futures internally ourselves?

We are in a few places.
While it is true that wasm_bindgen_futures::spawn_local() actually uses queueMicrotask() internally, it also does way more then just that, which we don't need in this case.

And why does web_sys not expose queueMicrotask?

Just missing, I will make a PR.

[madsmtm]

Just missing, I will make a PR.

Don't sweat it, we do this sort of thing all the time in Winit, just curious.

[daxpedda]

Just missing, I will make a PR.

Don't sweat it, we do this sort of thing all the time in Winit, just curious.

Done in rustwasm/wasm-bindgen#3981.