On Web, implement WindowEvent::Occluded (#2940)

src/platform_impl/web/web_sys/intersection_handle.rs

@@ -0,0 +1,45 @@
+use js_sys::Array;
+use wasm_bindgen::{prelude::Closure, JsCast};
+use web_sys::{Element, IntersectionObserver, IntersectionObserverEntry};
+
+pub(super) struct IntersectionObserverHandle {
+    observer: IntersectionObserver,
+    _closure: Closure<dyn FnMut(Array)>,
+}
+
+impl IntersectionObserverHandle {
+    pub fn new<F>(element: &Element, mut callback: F) -> Self
+    where
+        F: 'static + FnMut(bool),
+    {
+        let mut skip = true;
+        let closure = Closure::new(move |entries: Array| {
+            let entry: IntersectionObserverEntry = entries.get(0).unchecked_into();
+
+            let is_intersecting = entry.is_intersecting();
+
+            // skip first intersection
+            if skip && is_intersecting {
+                skip = false;
+                return;
+            }
+
+            callback(is_intersecting);
+        });
+        let observer = IntersectionObserver::new(closure.as_ref().unchecked_ref())
+            // we don't provide any `options`
+            .expect("Invalid `options`");
+        observer.observe(element);
+
+        Self {
+            observer,
+            _closure: closure,
+        }
+    }
+}
+
+impl Drop for IntersectionObserverHandle {
+    fn drop(&mut self) {
+        self.observer.disconnect()
+    }
+}