🤖 fix: scroll to bottom on workspace switch using ResizeObserver

The previous double requestAnimationFrame approach failed when async
content (Shiki highlighting, Mermaid diagrams, images) rendered after
the scroll completed. Now we observe the inner message container for
size changes during the auto-scroll window and re-scroll as needed.

Changes:
- Add ResizeObserver in useAutoScroll that triggers scroll on growth
- Return innerRef callback ref from hook for attaching to content div
- Wire up innerRef in AIView's message container

_Generated with `mux`_

Author: ammar-agent

src/browser/components/AIView.tsx

@@ -209,6 +209,7 @@ const AIViewInner: React.FC<AIViewProps> = ({
   // Use auto-scroll hook for scroll management
   const {
     contentRef,
+    innerRef,
     autoScroll,
     setAutoScroll,
     performAutoScroll,
@@ -500,7 +501,7 @@ const AIViewInner: React.FC<AIViewProps> = ({
             data-testid="message-window"
             className="h-full overflow-y-auto p-[15px] leading-[1.5] break-words whitespace-pre-wrap"
           >
-            <div className={cn("max-w-4xl mx-auto", mergedMessages.length === 0 && "h-full")}>
+            <div ref={innerRef} className={cn("max-w-4xl mx-auto", mergedMessages.length === 0 && "h-full")}>
               {mergedMessages.length === 0 ? (
                 <div className="text-placeholder flex h-full flex-1 flex-col items-center justify-center text-center [&_h3]:m-0 [&_h3]:mb-2.5 [&_h3]:text-base [&_h3]:font-medium [&_p]:m-0 [&_p]:text-[13px]">
                   <h3>No Messages Yet</h3>

src/browser/hooks/useAutoScroll.ts

@@ -1,7 +1,14 @@
 import { useRef, useState, useCallback } from "react";
 
 /**
- * Hook to manage auto-scrolling behavior for a scrollable container
+ * Hook to manage auto-scrolling behavior for a scrollable container.
+ *
+ * Scroll container structure expected:
+ *   <div ref={contentRef}>           ← scroll container (overflow-y: auto)
+ *     <div ref={innerRef}>           ← inner content wrapper (observed for size changes)
+ *       {children}
+ *     </div>
+ *   </div>
  *
  * Auto-scroll is enabled when:
  * - User sends a message
@@ -17,10 +24,35 @@ export function useAutoScroll() {
   const lastUserInteractionRef = useRef<number>(0);
   // Ref to avoid stale closures in async callbacks - always holds current autoScroll value
   const autoScrollRef = useRef<boolean>(true);
+  // Track the ResizeObserver so we can disconnect it when the element unmounts
+  const observerRef = useRef<ResizeObserver | null>(null);
 
   // Sync ref with state to ensure callbacks always have latest value
   autoScrollRef.current = autoScroll;
 
+  // Callback ref for the inner content wrapper - sets up ResizeObserver when element mounts.
+  // ResizeObserver fires when the content size changes (Shiki highlighting, Mermaid, images, etc.),
+  // allowing us to scroll to bottom even when async content renders after the initial mount.
+  const innerRef = useCallback((element: HTMLDivElement | null) => {
+    // Cleanup previous observer if any
+    if (observerRef.current) {
+      observerRef.current.disconnect();
+      observerRef.current = null;
+    }
+
+    if (!element) return;
+
+    const observer = new ResizeObserver(() => {
+      // Only auto-scroll if enabled - user may have scrolled up
+      if (autoScrollRef.current && contentRef.current) {
+        contentRef.current.scrollTop = contentRef.current.scrollHeight;
+      }
+    });
+
+    observer.observe(element);
+    observerRef.current = observer;
+  }, []);
+
   const performAutoScroll = useCallback(() => {
     if (!contentRef.current) return;
 
@@ -38,18 +70,14 @@ export function useAutoScroll() {
   }, []); // No deps - ref ensures we always check current value
 
   const jumpToBottom = useCallback(() => {
-    if (!contentRef.current) return;
-
-    // Double RAF: First frame for DOM updates (async highlighting, image loads),
-    // second frame to scroll after layout is complete
-    requestAnimationFrame(() => {
-      requestAnimationFrame(() => {
-        if (contentRef.current) {
-          contentRef.current.scrollTop = contentRef.current.scrollHeight;
-        }
-      });
-    });
+    // Enable auto-scroll first so ResizeObserver will handle subsequent changes
     setAutoScroll(true);
+    autoScrollRef.current = true;
+
+    // Immediate scroll for content that's already rendered
+    if (contentRef.current) {
+      contentRef.current.scrollTop = contentRef.current.scrollHeight;
+    }
   }, []);
 
   const handleScroll = useCallback((e: React.UIEvent<HTMLDivElement>) => {
@@ -73,9 +101,11 @@ export function useAutoScroll() {
     if (isScrollingUp) {
       // Always disable auto-scroll when scrolling up
       setAutoScroll(false);
+      autoScrollRef.current = false;
     } else if (isScrollingDown && isAtBottom) {
       // Only enable auto-scroll if scrolling down AND reached the bottom
       setAutoScroll(true);
+      autoScrollRef.current = true;
     }
     // If scrolling down but not at bottom, auto-scroll remains disabled
 
@@ -89,6 +119,7 @@ export function useAutoScroll() {
 
   return {
     contentRef,
+    innerRef,
     autoScroll,
     setAutoScroll,
     performAutoScroll,