Function cleanupExisting

src/useOnInView.tsx:72–78

()

      // React <19 never calls ref callbacks with `null` during unmount, so we
      // eagerly tear down existing observers manually whenever the target changes.
      const cleanupExisting = () => {
        if (observerCleanupRef.current) {
          const cleanup = observerCleanupRef.current;
          observerCleanupRef.current = undefined;
          cleanup();
        }
      };

      if (element === observedElementRef.current) {
        return observerCleanupRef.current;