Closes #22653 again

In PR #23283, I thought that every `runnable` dispatched to the main
thread would correspond to an `EVENT_DISPATCHED` message in the message
queue. However, after testing, some `runnable`s occasionally weren’t
executed.

This PR updated the code as follows:  
```rust
if let Ok(runnable) = self.main_receiver.try_recv() {    <-- before
for runnable in self.main_receiver.drain() {            <-- after
    runnable.run();
}
```

This ensures that runnables are handled more proactively on the main
thread, now we handle `runnable`s with a much higher priority.

A big thanks to @MolotovCherry and @ArthurBrussee for their testing
efforts!

Release Notes:

- N/A