Web: improve custom cursor handling and add animated cursors (#3384)

src/platform_impl/web/async/atomic_waker.rs

@@ -0,0 +1,35 @@
+use std::cell::RefCell;
+use std::ops::Deref;
+use std::task::Waker;
+
+#[derive(Debug)]
+pub struct AtomicWaker(RefCell<Option<Waker>>);
+
+impl AtomicWaker {
+    pub const fn new() -> Self {
+        Self(RefCell::new(None))
+    }
+
+    pub fn register(&self, waker: &Waker) {
+        let mut this = self.0.borrow_mut();
+
+        if let Some(old_waker) = this.deref() {
+            if old_waker.will_wake(waker) {
+                return;
+            }
+        }
+
+        *this = Some(waker.clone());
+    }
+
+    pub fn wake(&self) {
+        if let Some(waker) = self.0.borrow_mut().take() {
+            waker.wake();
+        }
+    }
+}
+
+// SAFETY: Wasm without the `atomics` target feature is single-threaded.
+unsafe impl Send for AtomicWaker {}
+// SAFETY: Wasm without the `atomics` target feature is single-threaded.
+unsafe impl Sync for AtomicWaker {}