Pre-fetch user events when possible

src/platform_impl/web/async/waker.rs

@@ -83,6 +83,16 @@ impl<T> WakerSpawner<T> {
     pub fn waker(&self) -> Waker<T> {
         Waker(self.0.clone())
     }
+
+    pub fn fetch(&self) -> usize {
+        debug_assert!(
+            self.0.is_main_thread(),
+            "this should only be called from the main thread"
+        );
+
+        self.0
+            .with_sender_data(|inner| inner.0.counter.swap(0, Ordering::Relaxed))
+    }
 }
 
 impl<T> Drop for WakerSpawner<T> {

src/platform_impl/web/event_loop/runner.rs

@@ -631,7 +631,18 @@ impl Shared {
         if !is_closed && self.0.runner.borrow().maybe_runner().is_some() {
             // Take an event out of the queue and handle it
             // Make sure not to let the borrow_mut live during the next handle_event
-            let event = { self.0.events.borrow_mut().pop_front() };
+            let event = {
+                let mut events = self.0.events.borrow_mut();
+
+                // Pre-fetch `UserEvent`s to avoid having to wait until the next event loop cycle.
+                events.extend(
+                    iter::repeat(Event::UserEvent(()))
+                        .take(self.0.proxy_spawner.fetch())
+                        .map(EventWrapper::from),
+                );
+
+                events.pop_front()
+            };
             if let Some(event) = event {
                 self.handle_event(event);
             }