feat(AIChatPanel): implement dynamic chat message font size and enhance user message scrolling behavior

src/renderer/components/ai/AIChatPanel.tsx

@@ -10,6 +10,9 @@ import { Skeleton } from '@/components/ui/skeleton';
 import { ScrollArea } from '@/components/ui/scroll-area';
 import { GradualBlur } from '@/components/ui/gradual-blur';
 
+/** Fluid font size for chat messages — scales with viewport width */
+const CHAT_FONT = 'clamp(1.125rem, 1.4vw, 1.375rem)';
+
 const SUGGESTION_CHIPS = [
   { icon: ListTodo, label: "What's on my plate today?" },
   { icon: TrendingUp, label: 'Summarize this week' },
@@ -78,17 +81,46 @@ export function AIChatPanel({
 
   const messagesContainerRef = useRef<HTMLDivElement | null>(null);
 
+  // --- Scroll-to-user-message + shrinking placeholder ---
+  const lastUserMsgRef = useRef<HTMLDivElement | null>(null);
+  const [streamingEl, setStreamingEl] = useState<HTMLDivElement | null>(null);
+  const [placeholderHeight, setPlaceholderHeight] = useState<number | null>(null);
+  const initialPlaceholderRef = useRef(0);
+  const pendingScrollRef = useRef(false);
+
   const briefMutation = trpc.ai.dailyBrief.useMutation();
 
-  const scrollToBottom = useCallback(() => {
-    const el = messagesContainerRef.current;
-    if (el) el.scrollTo({ top: el.scrollHeight });
-  }, []);
-
-  // Auto-scroll when messages change or streaming content updates
+  // When the user message appears in the list, set the placeholder and scroll it to the top
   useEffect(() => {
-    scrollToBottom();
-  }, [messages, streamingContent, scrollToBottom]);
+    if (!pendingScrollRef.current) return;
+    const lastMsg = messages[messages.length - 1];
+    if (!lastMsg || lastMsg.role !== 'user') return;
+
+    pendingScrollRef.current = false;
+    const ph = Math.round(window.innerHeight * 0.71);
+    initialPlaceholderRef.current = ph;
+    setPlaceholderHeight(ph);
+
+    // Double-rAF: wait for the placeholder div to actually paint before scrolling
+    requestAnimationFrame(() => {
+      requestAnimationFrame(() => {
+        lastUserMsgRef.current?.scrollIntoView({ block: 'start', behavior: 'smooth' });
+      });
+    });
+  }, [messages]);
+
+  // Shrink placeholder in real-time as AI streaming content grows
+  useEffect(() => {
+    if (!isStreaming || !streamingEl) return;
+    const MIN_PADDING = 80;
+    const observer = new ResizeObserver(() => {
+      const contentHeight = streamingEl.getBoundingClientRect().height;
+      setPlaceholderHeight(Math.max(MIN_PADDING, initialPlaceholderRef.current - contentHeight));
+    });
+    observer.observe(streamingEl);
+    return () => observer.disconnect();
+  }, [isStreaming, streamingEl]);
+
 
   // Auto-fire daily brief on home page
   useEffect(() => {
@@ -125,6 +157,7 @@ export function AIChatPanel({
 
   const handleSend = useCallback(() => {
     if (briefLoading) return;
+    pendingScrollRef.current = true;
     chatHandleSend();
   }, [briefLoading, chatHandleSend]);
 
@@ -324,12 +357,18 @@ export function AIChatPanel({
           <div className="mx-auto w-full max-w-6xl px-6 pt-8 pb-32">
             <div className="flex flex-col gap-4">
               {/* Chat messages */}
-              {messages.map((msg) => {
+              {messages.map((msg, idx) => {
+                const isLastMsg = idx === messages.length - 1;
+
                 if (msg.role === 'user') {
                   return (
-                    <div key={msg.id} className="flex justify-end">
+                    <div
+                      key={msg.id}
+                      ref={isLastMsg ? lastUserMsgRef : undefined}
+                      className="flex justify-end"
+                    >
                       <div className="ml-auto max-w-[75%] rounded-2xl bg-muted px-4 py-2">
-                        <ChatMarkdown content={msg.content} />
+                        <ChatMarkdown content={msg.content} fontSize={CHAT_FONT} />
                       </div>
                     </div>
                   );
@@ -338,7 +377,7 @@ export function AIChatPanel({
                 if (msg.error) {
                   return (
                     <div key={msg.id} className="mr-auto max-w-[75%]">
-                      <p className="text-sm text-destructive whitespace-pre-wrap">
+                      <p style={{ fontSize: CHAT_FONT }} className="text-destructive whitespace-pre-wrap">
                         {msg.content}
                       </p>
                     </div>
@@ -349,10 +388,10 @@ export function AIChatPanel({
                   <div key={msg.id} className="mr-auto max-w-[75%]">
                     <div className="flex items-center gap-1.5 mb-1">
                       <Sparkles size={16} className="text-foreground" />
-                      <span className="text-sm font-semibold">Adiuva</span>
+                      <span style={{ fontSize: CHAT_FONT }} className="font-semibold">Adiuva</span>
                     </div>
                     <div className="pl-[22px]">
-                      <ChatMarkdown content={msg.content} />
+                      <ChatMarkdown content={msg.content} fontSize={CHAT_FONT} />
                     </div>
                   </div>
                 );
@@ -360,14 +399,14 @@ export function AIChatPanel({
 
               {/* Streaming AI response */}
               {isStreaming && (
-                <div className="mr-auto max-w-[75%]">
+                <div ref={setStreamingEl} className="mr-auto max-w-[75%]">
                   <div className="flex items-center gap-1.5 mb-1">
                     <Sparkles size={16} className="text-foreground" />
-                    <span className="text-sm font-semibold">Adiuva</span>
+                    <span style={{ fontSize: CHAT_FONT }} className="font-semibold">Adiuva</span>
                   </div>
                   {streamingContent ? (
                     <div className="pl-[22px]">
-                      <ChatMarkdown content={streamingContent} />
+                      <ChatMarkdown content={streamingContent} fontSize={CHAT_FONT} />
                     </div>
                   ) : (
                     <div className="space-y-2 pl-[22px]">
@@ -377,6 +416,18 @@ export function AIChatPanel({
                   )}
                 </div>
               )}
+
+              {/* Placeholder: fills viewport after user message, shrinks as AI responds */}
+              {placeholderHeight !== null && (
+                <div
+                  aria-hidden
+                  style={{
+                    height: placeholderHeight,
+                    transition: 'height 180ms ease-out',
+                    flexShrink: 0,
+                  }}
+                />
+              )}
             </div>
           </div>
         )}
@@ -446,9 +497,12 @@ function ChatInput({
 
 /* ---------- ChatMarkdown: lightweight markdown renderer ---------- */
 
-export function ChatMarkdown({ content, size = 'sm' }: { content: string; size?: 'sm' | 'lg' }) {
+export function ChatMarkdown({ content, size = 'sm', fontSize }: { content: string; size?: 'sm' | 'lg'; fontSize?: string }) {
   return (
-    <div className={`prose dark:prose-invert max-w-none [&>*:first-child]:mt-0 [&>*:last-child]:mb-0 ${size === 'lg' ? 'prose-base' : 'prose-sm'}`}>
+    <div
+      className={`prose dark:prose-invert max-w-none [&>*:first-child]:mt-0 [&>*:last-child]:mb-0 ${size === 'lg' ? 'prose-base' : 'prose-sm'}`}
+      style={fontSize ? { fontSize } : undefined}
+    >
       <ReactMarkdown
         remarkPlugins={[remarkGfm]}
         components={{