feat(ui): add auto-scroll to bottom for chat messages

- Add useAutoScroll hook using ResizeObserver + MutationObserver
- Observes content children for size changes (streaming, images)
- Watches for new DOM nodes (new messages, history load)
- Respects user scroll position: no force-scroll when reading above
- Integrate in Chat component alongside existing useScrollFade

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/ui/src/components/chat.tsx

@@ -11,6 +11,7 @@ import { UserIcon, Copy01Icon, CheckmarkCircle02Icon } from "@hugeicons/core-fre
 import { toast } from "@multica/ui/components/ui/sonner";
 import { useHubStore, useDeviceId, useMessagesStore, useGatewayStore } from "@multica/store";
 import { useScrollFade } from "@multica/ui/hooks/use-scroll-fade";
+import { useAutoScroll } from "@multica/ui/hooks/use-auto-scroll";
 import { Skeleton } from "@multica/ui/components/ui/skeleton";
 import { cn } from "@multica/ui/lib/utils";
 
@@ -54,6 +55,7 @@ export function Chat() {
 
   const mainRef = useRef<HTMLElement>(null)
   const fadeStyle = useScrollFade(mainRef)
+  useAutoScroll(mainRef)
 
   return (
     <div className="h-dvh flex flex-col overflow-hidden w-full">

packages/ui/src/hooks/use-auto-scroll.ts

@@ -0,0 +1,64 @@
+import { type RefObject, useEffect, useRef } from "react"
+
+/**
+ * Auto-scrolls a scroll container to the bottom when its inner content grows,
+ * as long as the user hasn't scrolled up to read older content.
+ *
+ * Observes child element size changes via ResizeObserver on all children,
+ * plus MutationObserver for added/removed nodes. Works for new messages,
+ * history loads, streaming updates, and image loads.
+ */
+export function useAutoScroll(ref: RefObject<HTMLElement | null>) {
+  const stickRef = useRef(true)
+
+  useEffect(() => {
+    const el = ref.current
+    if (!el) return
+
+    const scrollToBottom = () => {
+      el.scrollTo({ top: el.scrollHeight })
+    }
+
+    const onScroll = () => {
+      const { scrollTop, scrollHeight, clientHeight } = el
+      stickRef.current = scrollHeight - scrollTop - clientHeight < 50
+    }
+
+    const onContentChange = () => {
+      if (stickRef.current) {
+        scrollToBottom()
+      }
+    }
+
+    // Watch child element resizes (content growth, image loads, streaming)
+    const ro = new ResizeObserver(onContentChange)
+    for (const child of el.children) {
+      ro.observe(child)
+    }
+
+    // Watch for added/removed child nodes (new messages rendered)
+    const mo = new MutationObserver((mutations) => {
+      // Also observe newly added elements
+      for (const mutation of mutations) {
+        for (const node of mutation.addedNodes) {
+          if (node instanceof Element) {
+            ro.observe(node)
+          }
+        }
+      }
+      onContentChange()
+    })
+    mo.observe(el, { childList: true, subtree: true })
+
+    el.addEventListener("scroll", onScroll, { passive: true })
+
+    // Initial scroll to bottom
+    scrollToBottom()
+
+    return () => {
+      el.removeEventListener("scroll", onScroll)
+      ro.disconnect()
+      mo.disconnect()
+    }
+  }, [ref])
+}