fix(chat): use the original floating window, not the rewritten one (MUL-4235)

Follow-up to the merged #5080, which shipped a hand-written, simplified
ChatWindow and lost the original's animations / drag-resize / expand-minimize.
The floating window is just a quick entry point — it should be the original
UI, not a rewrite.

- Restore chat-window.tsx, chat-fab.tsx, chat-resize-handles.tsx and
  use-chat-resize.ts verbatim from main (0-diff): motion animations, drag
  resize, expand/minimize and the session dropdown are back.
- Restore the empty_state.returning_subtitle + starter_prompts i18n keys the
  original window renders (V2 had dropped them); drop the now-unused
  window.open_full_tooltip key the rewrite added.
- Settings gating is unchanged: FloatingChat still wraps the original FAB +
  window, gated by floatingChatEnabled (default off) and hidden on /chat.

typecheck: core/views/web/desktop green. tests: chat + settings views 126 pass.
Co-authored-by: multica-agent <github@multica.ai>

packages/views/chat/components/chat-fab.tsx

@@ -16,12 +16,6 @@ import { useT } from "../../i18n";
 
 const logger = createLogger("chat.ui");
 
-/**
- * Minimised floating chat button. Toggles the {@link ChatWindow} overlay open.
- * Availability is gated one level up by {@link FloatingChat} (Settings
- * preference + route), so this component only concerns itself with the
- * open/closed transition and the unread / running affordances.
- */
 export function ChatFab() {
   const { t } = useT("chat");
   const wsId = useWorkspaceId();

packages/views/chat/components/chat-resize-handles.tsx

@@ -0,0 +1,34 @@
+"use client";
+
+import React from "react";
+
+type DragDir = "left" | "top" | "corner";
+
+interface ChatResizeHandlesProps {
+  onDragStart: (e: React.PointerEvent, dir: DragDir) => void;
+}
+
+export function ChatResizeHandles({ onDragStart }: ChatResizeHandlesProps) {
+  return (
+    <>
+      {/* Left edge — expands width when dragged left */}
+      <div
+        aria-hidden
+        onPointerDown={(e) => onDragStart(e, "left")}
+        className="absolute left-0 top-4 bottom-0 w-1 z-10 cursor-col-resize"
+      />
+      {/* Top edge — expands height when dragged up */}
+      <div
+        aria-hidden
+        onPointerDown={(e) => onDragStart(e, "top")}
+        className="absolute top-0 left-4 right-0 h-1 z-10 cursor-row-resize"
+      />
+      {/* Top-left corner — expands both width and height */}
+      <div
+        aria-hidden
+        onPointerDown={(e) => onDragStart(e, "corner")}
+        className="absolute top-0 left-0 size-4 z-20 cursor-nw-resize"
+      />
+    </>
+  );
+}

packages/views/chat/components/use-chat-resize.ts

@@ -0,0 +1,140 @@
+"use client";
+
+import React, { useRef, useCallback, useState, useEffect } from "react";
+import { CHAT_MIN_W, CHAT_MIN_H, useChatStore } from "@multica/core/chat";
+
+type DragDir = "left" | "top" | "corner";
+
+const MAX_RATIO = 0.9;
+const FALLBACK_MAX_W = 800;
+const FALLBACK_MAX_H = 700;
+
+function clamp(v: number, min: number, max: number) {
+  return Math.max(min, Math.min(max, v));
+}
+
+export function useChatResize(
+  windowRef: React.RefObject<HTMLDivElement | null>,
+) {
+  const chatWidth = useChatStore((s) => s.chatWidth);
+  const chatHeight = useChatStore((s) => s.chatHeight);
+  const isExpanded = useChatStore((s) => s.isExpanded);
+  const setChatSize = useChatStore((s) => s.setChatSize);
+  const setExpanded = useChatStore((s) => s.setExpanded);
+
+  // ── Container bounds via ResizeObserver ────────────────────────────────
+  const boundsRef = useRef({ maxW: FALLBACK_MAX_W, maxH: FALLBACK_MAX_H });
+  const [boundsReady, setBoundsReady] = useState(false);
+  const [isDragging, setIsDragging] = useState(false);
+  const [, setRevision] = useState(0);
+
+  useEffect(() => {
+    const el = windowRef.current;
+    const parent = el?.parentElement;
+    if (!parent) return;
+
+    const update = () => {
+      const maxW = Math.floor(parent.clientWidth * MAX_RATIO);
+      const maxH = Math.floor(parent.clientHeight * MAX_RATIO);
+      setBoundsReady(true); // idempotent once true
+      // Only trigger a re-render if the bounds actually changed. Without this
+      // guard, any spurious ResizeObserver notification (including sub-pixel
+      // layout jitter during mount) schedules a setState that feeds back into
+      // the observer, producing "Maximum update depth exceeded".
+      const prev = boundsRef.current;
+      if (prev.maxW === maxW && prev.maxH === maxH) return;
+      boundsRef.current = { maxW, maxH };
+      setRevision((r) => r + 1);
+    };
+
+    // Measure immediately (parent is already in DOM at this point)
+    update();
+
+    const ro = new ResizeObserver(update);
+    ro.observe(parent);
+    return () => ro.disconnect();
+  }, [windowRef]);
+
+  // ── Derive rendered size ──────────────────────────────────────────────
+  const { maxW, maxH } = boundsRef.current;
+
+  const renderWidth = isExpanded ? maxW : clamp(chatWidth, CHAT_MIN_W, maxW);
+  const renderHeight = isExpanded ? maxH : clamp(chatHeight, CHAT_MIN_H, maxH);
+
+  // ── Expand / Restore ──────────────────────────────────────────────────
+  const isAtMax = renderWidth >= maxW && renderHeight >= maxH;
+
+  const toggleExpand = useCallback(() => {
+    if (isExpanded || isAtMax) {
+      setChatSize(CHAT_MIN_W, CHAT_MIN_H);
+    } else {
+      setExpanded(true);
+    }
+  }, [isExpanded, isAtMax, setChatSize, setExpanded]);
+
+  // ── Drag ──────────────────────────────────────────────────────────────
+  const dragRef = useRef<{
+    startX: number;
+    startY: number;
+    startW: number;
+    startH: number;
+    dir: DragDir;
+  } | null>(null);
+
+  const startDrag = useCallback(
+    (e: React.PointerEvent, dir: DragDir) => {
+      e.preventDefault();
+      (e.target as HTMLElement).setPointerCapture(e.pointerId);
+
+      dragRef.current = {
+        startX: e.clientX,
+        startY: e.clientY,
+        startW: renderWidth,
+        startH: renderHeight,
+        dir,
+      };
+      setIsDragging(true);
+
+      const onPointerMove = (ev: PointerEvent) => {
+        const d = dragRef.current;
+        if (!d) return;
+
+        const { maxW: mw, maxH: mh } = boundsRef.current;
+
+        const rawW =
+          dir === "left" || dir === "corner"
+            ? d.startW - (ev.clientX - d.startX)
+            : d.startW;
+        const rawH =
+          dir === "top" || dir === "corner"
+            ? d.startH - (ev.clientY - d.startY)
+            : d.startH;
+
+        setChatSize(clamp(rawW, CHAT_MIN_W, mw), clamp(rawH, CHAT_MIN_H, mh));
+      };
+
+      const onPointerUp = () => {
+        dragRef.current = null;
+        setIsDragging(false);
+        document.removeEventListener("pointermove", onPointerMove);
+        document.removeEventListener("pointerup", onPointerUp);
+        document.body.style.cursor = "";
+        document.body.style.userSelect = "";
+      };
+
+      document.addEventListener("pointermove", onPointerMove);
+      document.addEventListener("pointerup", onPointerUp);
+
+      const cursorMap: Record<DragDir, string> = {
+        left: "col-resize",
+        top: "row-resize",
+        corner: "nw-resize",
+      };
+      document.body.style.cursor = cursorMap[dir];
+      document.body.style.userSelect = "none";
+    },
+    [renderWidth, renderHeight, setChatSize],
+  );
+
+  return { renderWidth, renderHeight, isAtMax, boundsReady, isDragging, toggleExpand, startDrag };
+}

packages/views/locales/en/chat.json

@@ -72,7 +72,6 @@
     }
   },
   "window": {
-    "open_full_tooltip": "Open full page",
     "new_chat_tooltip": "New chat",
     "restore_tooltip": "Restore",
     "expand_tooltip": "Expand",
@@ -97,8 +96,14 @@
     "first_time_actions": "Ask for a summary, plan your day, or hand off a quick task.",
     "returning_title_named": "Hi, I'm {{name}}",
     "returning_title_default": "Welcome to Multica",
+    "returning_subtitle": "Try asking",
     "chat_with_named": "Chat with {{name}}"
   },
+  "starter_prompts": {
+    "list_open": "List my open tasks by priority",
+    "summarize_today": "Summarize what I did today",
+    "plan_next": "Plan what to work on next"
+  },
   "no_agent_banner": "You need an agent to start chatting.",
   "offline_banner": {
     "fallback_name": "the agent",

packages/views/locales/ja/chat.json

@@ -69,7 +69,6 @@
     }
   },
   "window": {
-    "open_full_tooltip": "全画面で開く",
     "new_chat_tooltip": "新規チャット",
     "restore_tooltip": "復元",
     "expand_tooltip": "展開",
@@ -94,8 +93,14 @@
     "first_time_actions": "要約を頼んだり、今日の予定を立てたり、簡単なタスクを任せたりしてみましょう。",
     "returning_title_named": "こんにちは、{{name}} です",
     "returning_title_default": "Multica へようこそ",
+    "returning_subtitle": "こう聞いてみましょう",
     "chat_with_named": "{{name}} とチャット"
   },
+  "starter_prompts": {
+    "list_open": "オープンなタスクを優先度順にまとめて",
+    "summarize_today": "今日やったことを要約して",
+    "plan_next": "次に取り組むことを計画して"
+  },
   "no_agent_banner": "チャットを始めるにはエージェントが必要です。",
   "offline_banner": {
     "fallback_name": "エージェント",

packages/views/locales/ko/chat.json

@@ -69,7 +69,6 @@
     }
   },
   "window": {
-    "open_full_tooltip": "전체 페이지 열기",
     "new_chat_tooltip": "새 채팅",
     "restore_tooltip": "복원",
     "expand_tooltip": "펼치기",
@@ -94,8 +93,14 @@
     "first_time_actions": "요약을 요청하거나, 오늘 할 일을 계획하거나, 간단한 작업을 맡겨보세요.",
     "returning_title_named": "안녕하세요, 저는 {{name}}입니다",
     "returning_title_default": "Multica에 오신 것을 환영합니다",
+    "returning_subtitle": "이렇게 요청해 보세요",
     "chat_with_named": "{{name}}와 채팅"
   },
+  "starter_prompts": {
+    "list_open": "열린 작업을 우선순위별로 정리해 줘",
+    "summarize_today": "오늘 내가 한 일을 요약해 줘",
+    "plan_next": "다음에 할 일을 계획해 줘"
+  },
   "no_agent_banner": "채팅을 시작하려면 에이전트가 필요합니다.",
   "offline_banner": {
     "fallback_name": "에이전트",

packages/views/locales/zh-Hans/chat.json

@@ -69,7 +69,6 @@
     }
   },
   "window": {
-    "open_full_tooltip": "打开完整页面",
     "new_chat_tooltip": "新对话",
     "restore_tooltip": "还原",
     "expand_tooltip": "展开",
@@ -94,8 +93,14 @@
     "first_time_actions": "让它做一份总结、规划你的一天，或交给它一个小任务。",
     "returning_title_named": "你好，我是 {{name}}",
     "returning_title_default": "欢迎使用 Multica",
+    "returning_subtitle": "试试问",
     "chat_with_named": "与 {{name}} 对话"
   },
+  "starter_prompts": {
+    "list_open": "按优先级列出我未完成的任务",
+    "summarize_today": "总结一下我今天做了什么",
+    "plan_next": "规划接下来该做什么"
+  },
   "no_agent_banner": "需要先有一个智能体才能开始对话。",
   "offline_banner": {
     "fallback_name": "智能体",