chore: import upstream snapshot with attribution

apps/webapp/app/hooks/useElementVisibility.ts

@@ -0,0 +1,33 @@
+import { useEffect, useRef } from "react";
+
+type UseElementVisibilityOptions = {
+  onVisibilityChange?: (isVisible: boolean) => void;
+};
+
+export function useElementVisibility({ onVisibilityChange }: UseElementVisibilityOptions = {}) {
+  const ref = useRef<HTMLDivElement>(null);
+  const isVisibleRef = useRef(false);
+  const callbackRef = useRef(onVisibilityChange);
+  callbackRef.current = onVisibilityChange;
+
+  useEffect(() => {
+    const el = ref.current;
+    if (!el) return;
+
+    const observer = new IntersectionObserver(
+      ([entry]) => {
+        const nowVisible = entry.isIntersecting;
+        if (isVisibleRef.current !== nowVisible) {
+          isVisibleRef.current = nowVisible;
+          callbackRef.current?.(nowVisible);
+        }
+      },
+      { threshold: 0 }
+    );
+
+    observer.observe(el);
+    return () => observer.disconnect();
+  }, []);
+
+  return { ref, isVisibleRef };
+}