fix(web): artifact 放大窗 portal 到 body — 小窗 transform 祖先把 fixed 圈住致其困在助手窗内

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/pac-web/src/components/assistant/assistant-chat.tsx

 'use client';
 
 import { useEffect, useMemo, useRef, useState } from 'react';
+import { createPortal } from 'react-dom';
 import ReactMarkdown from 'react-markdown';
 import remarkGfm from 'remark-gfm';
 import {
@@ -349,8 +350,12 @@ function ArtifactView({ artifact }: { artifact: Artifact }) {
       </div>
       <ArtifactFrame artifact={artifact} autoHeight />
 
-      {/* 居中放大窗:与内联卡同源同流(streaming 时这里同步在"长") */}
-      {expanded && (
+      {/* 居中放大窗:与内联卡同源同流(streaming 时这里同步在"长")。
+          ⚠️ portal 到 body —— 助手小窗容器带 transform(滑出动画),CSS 规定 transform 祖先
+          会把后代 fixed 圈进自己坐标系 → 不 portal 大窗就被困在小窗里(2026-06 实测)。 */}
+      {expanded &&
+        typeof document !== 'undefined' &&
+        createPortal(
         <div className="fixed inset-0 z-[70] flex items-center justify-center p-4">
           <div className="absolute inset-0 bg-black/40" onClick={() => setExpanded(false)} />
           <div className="relative z-10 flex h-[min(86vh,920px)] w-[min(980px,94vw)] flex-col overflow-hidden rounded-xl border border-slate-200 bg-white shadow-2xl">
@@ -382,7 +387,8 @@ function ArtifactView({ artifact }: { artifact: Artifact }) {
             </div>
             <ArtifactFrame artifact={artifact} autoHeight={false} className="min-h-0 flex-1" />
           </div>
-        </div>
+        </div>,
+        document.body,
       )}
     </div>
   );