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

src/platform_impl/web/async/concurrent_queue.rs

@@ -0,0 +1,55 @@
+use std::cell::{Cell, RefCell};
+
+#[derive(Debug)]
+pub struct ConcurrentQueue<T> {
+    queue: RefCell<Vec<T>>,
+    closed: Cell<bool>,
+}
+
+pub enum PushError<T> {
+    #[allow(dead_code)]
+    Full(T),
+    Closed(T),
+}
+
+pub enum PopError {
+    Empty,
+    Closed,
+}
+
+impl<T> ConcurrentQueue<T> {
+    pub fn unbounded() -> Self {
+        Self {
+            queue: RefCell::new(Vec::new()),
+            closed: Cell::new(false),
+        }
+    }
+
+    pub fn push(&self, value: T) -> Result<(), PushError<T>> {
+        if self.closed.get() {
+            return Err(PushError::Closed(value));
+        }
+
+        self.queue.borrow_mut().push(value);
+        Ok(())
+    }
+
+    pub fn pop(&self) -> Result<T, PopError> {
+        self.queue.borrow_mut().pop().ok_or_else(|| {
+            if self.closed.get() {
+                PopError::Closed
+            } else {
+                PopError::Empty
+            }
+        })
+    }
+
+    pub fn close(&self) -> bool {
+        !self.closed.replace(true)
+    }
+}
+
+// SAFETY: Wasm without the `atomics` target feature is single-threaded.
+unsafe impl<T> Send for ConcurrentQueue<T> {}
+// SAFETY: Wasm without the `atomics` target feature is single-threaded.
+unsafe impl<T> Sync for ConcurrentQueue<T> {}