feat: chat autoscroll (#6)

packages/bolt/app/lib/hooks/useSnapScroll.ts

@@ -0,0 +1,54 @@
+import { useRef, useCallback } from 'react';
+
+export function useSnapScroll() {
+  const autoScrollRef = useRef(true);
+  const scrollNodeRef = useRef<HTMLDivElement>();
+  const onScrollRef = useRef<() => void>();
+  const observerRef = useRef<ResizeObserver>();
+
+  const messageRef = useCallback((node: HTMLDivElement | null) => {
+    if (node) {
+      const observer = new ResizeObserver(() => {
+        if (autoScrollRef.current) {
+          if (scrollNodeRef.current) {
+            const { scrollHeight, clientHeight } = scrollNodeRef.current;
+            const scrollTarget = scrollHeight - clientHeight;
+
+            scrollNodeRef.current.scrollTo({
+              top: scrollTarget,
+            });
+          }
+        }
+      });
+
+      observer.observe(node);
+    } else {
+      observerRef.current?.disconnect();
+      observerRef.current = undefined;
+    }
+  }, []);
+
+  const scrollRef = useCallback((node: HTMLDivElement | null) => {
+    if (node) {
+      onScrollRef.current = () => {
+        const { scrollTop, scrollHeight, clientHeight } = node;
+        const scrollTarget = scrollHeight - clientHeight;
+
+        autoScrollRef.current = Math.abs(scrollTop - scrollTarget) <= 10;
+      };
+
+      node.addEventListener('scroll', onScrollRef.current);
+
+      scrollNodeRef.current = node;
+    } else {
+      if (onScrollRef.current) {
+        scrollNodeRef.current?.removeEventListener('scroll', onScrollRef.current);
+      }
+
+      scrollNodeRef.current = undefined;
+      onScrollRef.current = undefined;
+    }
+  }, []);
+
+  return [messageRef, scrollRef];
+}