diff --git a/packages/dashboard/app/App.tsx b/packages/dashboard/app/App.tsx
index 5dfd2474d6..817ac09686 100644
--- a/packages/dashboard/app/App.tsx
+++ b/packages/dashboard/app/App.tsx
@@ -568,6 +568,27 @@ function AppInner() {
   }, [initialLoadComplete]);
 
   const [quickChatOpen, setQuickChatOpen] = useState(false);
+  const [quickChatEverOpenedProjectId, setQuickChatEverOpenedProjectId] = useState<string | null>(null);
+  const quickChatProjectIdRef = useRef<string | undefined>(undefined);
+
+  /*
+  FNXC:ChatModal 2026-07-18-00:00:
+  FN-8257 requires Quick Chat to mount only after its first open, then remain mounted and hidden
+  across close/reopen so ChatView retains its selected session, transcript, and scroll position.
+  Reset the latch when the project changes so a hidden ChatView cannot leak one project's state
+  into another project; the FloatingWindow key supplies the matching React identity boundary.
+  */
+  useEffect(() => {
+    const projectId = currentProject?.id;
+    if (quickChatProjectIdRef.current !== projectId) {
+      quickChatProjectIdRef.current = projectId;
+      setQuickChatEverOpenedProjectId(quickChatOpen && projectId ? projectId : null);
+      return;
+    }
+    if (quickChatOpen && projectId) {
+      setQuickChatEverOpenedProjectId(projectId);
+    }
+  }, [currentProject?.id, quickChatOpen]);
 
   const { keyboardOpen } = useMobileKeyboard({ enabled: isMobile });
   // Keyboard visibility controls both MobileNavBar rendering and whether
@@ -1829,9 +1850,11 @@ function AppInner() {
           onOpenChange={setQuickChatOpen}
         />
       )}
-      {quickChatOpen && currentProject && (
+      {currentProject && quickChatEverOpenedProjectId === currentProject.id && (
         <FloatingWindow
+          key={currentProject.id}
           windowKey="chat-modal"
+          hidden={!quickChatOpen}
           title="Chat"
           onClose={() => setQuickChatOpen(false)}
           closeOnOutsidePointerDown={quickChatCloseOnOutsideClick}
diff --git a/packages/dashboard/app/components/FloatingWindow.css b/packages/dashboard/app/components/FloatingWindow.css
index 4416c193e9..6b36adcac2 100644
--- a/packages/dashboard/app/components/FloatingWindow.css
+++ b/packages/dashboard/app/components/FloatingWindow.css
@@ -13,6 +13,16 @@ Click-through overlays cannot implement backdrop clicks in CSS/DOM structure. Ou
   pointer-events: none;
 }
 
+/*
+FNXC:FloatingWindow 2026-07-18-00:00:
+Quick Chat closes by hiding its already-mounted portal instead of unmounting ChatView. `display:
+none` removes the desktop window and the ≤768px full-screen sheet from paint and interaction while
+preserving the child DOM and in-memory scroll/session state for an instant reopen.
+*/
+.floating-window-overlay--hidden {
+  display: none;
+}
+
 /*
 FNXC:FloatingWindow 2026-06-22-20:45:
 Floating panel positioned by state-driven inline `left/top/width/height` and stacked by inline `z-index`. min/max keep the panel usable and on-screen. `resize: none` because resizing is handled by the corner/edge handles. `pointer-events: auto` re-enables interaction on the panel only.
diff --git a/packages/dashboard/app/components/FloatingWindow.tsx b/packages/dashboard/app/components/FloatingWindow.tsx
index b399f7bea9..9f805db8b4 100644
--- a/packages/dashboard/app/components/FloatingWindow.tsx
+++ b/packages/dashboard/app/components/FloatingWindow.tsx
@@ -58,6 +58,13 @@ export interface FloatingWindowProps {
    * Persistent task/terminal pop-outs must omit this so page clicks do not close them.
    */
   closeOnOutsidePointerDown?: boolean;
+  /*
+  FNXC:FloatingWindow 2026-07-18-00:00:
+  Quick Chat must hide without unmounting so its active session, messages, and scroll position
+  reopen instantly. This opt-in flag keeps children mounted but removes the window from paint and
+  interaction; defaulting to false preserves every existing FloatingWindow caller unchanged.
+  */
+  hidden?: boolean;
   /**
    * Layer band for z-index claiming. Task-detail peers (including Quick Chat) interleave by
    * interaction; unrelated utilities use the global floating stack.
@@ -186,6 +193,7 @@ export function FloatingWindow({
   suspendGeometryPersistenceOnMobile = false,
   suspendGeometryPersistenceOnShortViewport = false,
   closeOnOutsidePointerDown = false,
+  hidden = false,
   layer = "utility",
   ariaLabel,
 }: FloatingWindowProps) {
@@ -239,6 +247,16 @@ export function FloatingWindow({
     });
   }, [claimFrontZ, readCurrentZ]);
 
+  /*
+  FNXC:FloatingWindow 2026-07-18-00:00:
+  A hidden Quick Chat must reclaim a fresh z-index when reopened because another task-detail
+  popup may have been focused while chat was invisible. Hidden windows do not otherwise affect
+  the shared interaction stack.
+  */
+  useEffect(() => {
+    if (!hidden) bringToFront();
+  }, [bringToFront, hidden]);
+
   const handleDragPointerDown = useCallback(
     (event: ReactPointerEvent<HTMLDivElement>) => {
       if ((event.target as HTMLElement).closest("button")) return;
@@ -384,16 +402,16 @@ export function FloatingWindow({
   Root-portaled Activity menus cannot inherit movement from a dragged/resized task popup. Emit a bounded geometry-change signal after FloatingWindow commits new geometry so owning task-detail content can recompute fixed menu coordinates from the live Activity trigger rect.
   */
   useLayoutEffect(() => {
-    if (typeof window === "undefined") return;
+    if (hidden || typeof window === "undefined") return;
     window.dispatchEvent(new CustomEvent(FLOATING_WINDOW_GEOMETRY_CHANGE_EVENT, { detail: { windowKey, layer } }));
-  }, [layer, position, size, windowKey]);
+  }, [hidden, layer, position, size, windowKey]);
 
   /*
   FNXC:FloatingWindow 2026-06-27-00:00:
   Outside-click dismissal is opt-in because the overlay is intentionally click-through for coexisting floating windows. A capture-phase document pointerdown listener is the only reliable outside signal, and it must ignore in-flight drag/resize gestures plus nested modal/floating surfaces so Quick Chat can dismiss from bare-page clicks without making persistent task pop-outs fragile.
   */
   useEffect(() => {
-    if (!closeOnOutsidePointerDown || typeof document === "undefined") return;
+    if (hidden || !closeOnOutsidePointerDown || typeof document === "undefined") return;
 
     let lastTouchAt = 0;
     const markTouch = () => {
@@ -423,20 +441,20 @@ export function FloatingWindow({
       document.removeEventListener("touchend", markTouch);
       document.removeEventListener("pointerdown", handleDocumentPointerDown, true);
     };
-  }, [closeOnOutsidePointerDown, onClose]);
+  }, [closeOnOutsidePointerDown, hidden, onClose]);
 
   /*
   FNXC:ChatModal 2026-06-22-14:57:
   Quick Chat reopens should restore the last desktop floating-window size and position while still clamping onto the current viewport. Keep persistence generic and opt-in with persistGeometryKey so each caller controls whether geometry is shared or isolated.
   */
   useEffect(() => {
-    if (!persistGeometryKey || typeof window === "undefined" || geometryPersistenceSuspended) return;
+    if (hidden || !persistGeometryKey || typeof window === "undefined" || geometryPersistenceSuspended) return;
     try {
       localStorage.setItem(persistGeometryKey, JSON.stringify({ size, position }));
     } catch {
       // Ignore storage failures; geometry persistence is a convenience only.
     }
-  }, [geometryPersistenceSuspended, persistGeometryKey, position, size]);
+  }, [geometryPersistenceSuspended, hidden, persistGeometryKey, position, size]);
 
   const panelStyle = {
     left: `${position.x}px`,
@@ -449,12 +467,18 @@ export function FloatingWindow({
   /*
   FNXC:FloatingWindow 2026-06-22-21:10:
   Rendered via a portal to document.body so the window escapes every ancestor stacking context (board card badges, the List view's sticky sort header + column divider, transformed columns, etc.). Without the portal the panel's z-index battles inside whatever subtree mounted it, letting card dependency/overlap tags and the list divider/sort header paint over the modal. At document.body the 4000+ z-index wins over all page content.
+
+  FNXC:FloatingWindow 2026-07-18-00:00:
+  Hidden windows remain portaled so their child component identity survives a close/reopen cycle.
+  The hidden overlay is display:none and aria-hidden, which removes it from paint, focus, and
+  pointer interaction while the effects above suspend invisible-window side effects.
   */
   return createPortal(
     <div
-      className="floating-window-overlay"
+      className={`floating-window-overlay${hidden ? " floating-window-overlay--hidden" : ""}`}
       role="dialog"
       aria-modal="false"
+      aria-hidden={hidden || undefined}
       aria-label={ariaLabel}
       data-testid={`floating-window-overlay-${windowKey}`}
       // FNXC:FloatingWindow 2026-06-22-23:00: The z-index MUST live on the position:fixed overlay (which creates a stacking context), not the panel. A panel z-index is trapped inside the overlay's context and loses to page elements that are stacking contexts in body's context (e.g. the right dock at position:absolute z-index:20). With z on the overlay, the whole window sits at the shared floating band in body's stacking context and reliably paints above page content + tap-to-front reorders correctly.
diff --git a/packages/dashboard/app/components/__tests__/FloatingWindow.test.tsx b/packages/dashboard/app/components/__tests__/FloatingWindow.test.tsx
index 6dde8853f2..9abb92cc1b 100644
--- a/packages/dashboard/app/components/__tests__/FloatingWindow.test.tsx
+++ b/packages/dashboard/app/components/__tests__/FloatingWindow.test.tsx
@@ -2,7 +2,7 @@ import { render, screen, fireEvent } from "@testing-library/react";
 import { readFileSync } from "node:fs";
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import { loadAllAppCss, loadStylesCss } from "../../test/cssFixture";
-import { FloatingWindow } from "../FloatingWindow";
+import { FLOATING_WINDOW_GEOMETRY_CHANGE_EVENT, FloatingWindow } from "../FloatingWindow";
 
 const floatingWindowCss = readFileSync("app/components/FloatingWindow.css", "utf8");
 const allAppCss = loadAllAppCss();
@@ -851,6 +851,94 @@ describe("FloatingWindow", () => {
     expect(chatPanel.style.top).toBe("140px");
   });
 
+  it("keeps hidden children mounted while suspending invisible-window effects and reclaiming the task-detail stack", () => {
+    const onClose = vi.fn();
+    const geometryEvents = vi.fn();
+    const storageKey = "floating-window:hidden";
+    window.addEventListener(FLOATING_WINDOW_GEOMETRY_CHANGE_EVENT, geometryEvents);
+
+    const { rerender } = render(
+      <>
+        <FloatingWindow
+          windowKey="hidden-chat"
+          title="Chat"
+          onClose={onClose}
+          hidden
+          closeOnOutsidePointerDown
+          persistGeometryKey={storageKey}
+          layer="task-detail"
+        >
+          <div data-testid="retained-hidden-child">retained chat</div>
+        </FloatingWindow>
+        <FloatingWindow windowKey="active-task" title="Task" onClose={() => {}} layer="task-detail">
+          <div>active task</div>
+        </FloatingWindow>
+      </>,
+    );
+
+    const hiddenOverlay = screen.getByTestId("floating-window-overlay-hidden-chat");
+    const retainedChild = screen.getByTestId("retained-hidden-child");
+    const activeTask = screen.getByTestId("floating-window-active-task");
+    expect(hiddenOverlay).toHaveClass("floating-window-overlay--hidden");
+    expect(hiddenOverlay).toHaveAttribute("aria-hidden", "true");
+    expect(geometryEvents).toHaveBeenCalledTimes(1);
+    expect(localStorage.getItem(storageKey)).toBeNull();
+    fireEvent.pointerDown(document.body);
+    expect(onClose).not.toHaveBeenCalled();
+
+    rerender(
+      <>
+        <FloatingWindow
+          windowKey="hidden-chat"
+          title="Chat"
+          onClose={onClose}
+          closeOnOutsidePointerDown
+          persistGeometryKey={storageKey}
+          layer="task-detail"
+        >
+          <div data-testid="retained-hidden-child">retained chat</div>
+        </FloatingWindow>
+        <FloatingWindow windowKey="active-task" title="Task" onClose={() => {}} layer="task-detail">
+          <div>active task</div>
+        </FloatingWindow>
+      </>,
+    );
+
+    const visibleOverlay = screen.getByTestId("floating-window-overlay-hidden-chat");
+    const shownChat = screen.getByTestId("floating-window-hidden-chat");
+    expect(visibleOverlay).not.toHaveClass("floating-window-overlay--hidden");
+    expect(visibleOverlay).not.toHaveAttribute("aria-hidden");
+    expect(screen.getByTestId("retained-hidden-child")).toBe(retainedChild);
+    expect(geometryEvents).toHaveBeenCalledTimes(2);
+    expect(localStorage.getItem(storageKey)).not.toBeNull();
+    expect(Number(shownChat.style.zIndex)).toBeGreaterThan(Number(activeTask.style.zIndex));
+    fireEvent.pointerDown(document.body);
+    expect(onClose).toHaveBeenCalledTimes(1);
+    window.removeEventListener(FLOATING_WINDOW_GEOMETRY_CHANGE_EVENT, geometryEvents);
+  });
+
+  it("keeps the hidden prop opt-in so existing callers retain visible, interactive behavior", () => {
+    const onClose = vi.fn();
+    const geometryEvents = vi.fn();
+    const storageKey = "floating-window:default-hidden-off";
+    window.addEventListener(FLOATING_WINDOW_GEOMETRY_CHANGE_EVENT, geometryEvents);
+
+    render(
+      <FloatingWindow windowKey="default-hidden-off" title="Visible by default" onClose={onClose} closeOnOutsidePointerDown persistGeometryKey={storageKey}>
+        <div>existing caller body</div>
+      </FloatingWindow>,
+    );
+
+    const overlay = screen.getByTestId("floating-window-overlay-default-hidden-off");
+    expect(overlay).not.toHaveClass("floating-window-overlay--hidden");
+    expect(overlay).not.toHaveAttribute("aria-hidden");
+    expect(geometryEvents).toHaveBeenCalledTimes(1);
+    expect(localStorage.getItem(storageKey)).not.toBeNull();
+    fireEvent.pointerDown(document.body);
+    expect(onClose).toHaveBeenCalledTimes(1);
+    window.removeEventListener(FLOATING_WINDOW_GEOMETRY_CHANGE_EVENT, geometryEvents);
+  });
+
   it("makes only the mobile chat floating window full-screen", () => {
     const mobileBlock = floatingWindowCss.match(/@media\s*\(max-width:\s*768px\)\s*\{[\s\S]*?\.floating-window--chat \.chat-view\s*\{[\s\S]*?\n\}/)?.[0];
 
diff --git a/packages/dashboard/app/components/__tests__/QuickChat.persist.test.tsx b/packages/dashboard/app/components/__tests__/QuickChat.persist.test.tsx
new file mode 100644
index 0000000000..49096782bd
--- /dev/null
+++ b/packages/dashboard/app/components/__tests__/QuickChat.persist.test.tsx
@@ -0,0 +1,120 @@
+import { fireEvent, render, screen, waitFor } from "@testing-library/react";
+import { useEffect, useRef, useState } from "react";
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { FloatingWindow } from "../FloatingWindow";
+
+interface QuickChatHarnessProps {
+  projectId?: string;
+  onChatMount: () => void;
+}
+
+/*
+FNXC:ChatModal 2026-07-18-00:00:
+This focused App-shaped harness retains the production lifecycle boundary: Quick Chat lazily mounts
+only after the first open, then passes `hidden={!open}` to FloatingWindow. Keeping the test at this
+seam proves the user-visible close/reopen symptom without loading App's unrelated dashboard data.
+*/
+function QuickChatHarness({ projectId, onChatMount }: QuickChatHarnessProps) {
+  const [open, setOpen] = useState(false);
+  const [everOpenedProjectId, setEverOpenedProjectId] = useState<string | null>(null);
+  const trackedProjectId = useRef<string | undefined>(undefined);
+
+  useEffect(() => {
+    if (trackedProjectId.current !== projectId) {
+      trackedProjectId.current = projectId;
+      setEverOpenedProjectId(open && projectId ? projectId : null);
+      return;
+    }
+    if (open && projectId) setEverOpenedProjectId(projectId);
+  }, [open, projectId]);
+
+  return (
+    <>
+      <button type="button" onClick={() => setOpen(true)}>Open Quick Chat</button>
+      <button type="button" onClick={() => setOpen(false)}>Close Quick Chat</button>
+      {projectId && everOpenedProjectId === projectId && (
+        <FloatingWindow key={projectId} windowKey="quick-chat-persist" title="Chat" hidden={!open} onClose={() => setOpen(false)} className="floating-window--chat" suspendGeometryPersistenceOnMobile>
+          <RetainedChatProbe projectId={projectId} onMount={onChatMount} />
+        </FloatingWindow>
+      )}
+    </>
+  );
+}
+
+function RetainedChatProbe({ projectId, onMount }: { projectId: string; onMount: () => void }) {
+  const [session, setSession] = useState("Session one");
+  useEffect(() => onMount(), [onMount]);
+  return (
+    <div data-testid="quick-chat-body" data-project-id={projectId}>
+      <span data-testid="quick-chat-session">{session}</span>
+      <button type="button" onClick={() => setSession("Session two")}>Change session</button>
+      <div data-testid="quick-chat-scroll" />
+    </div>
+  );
+}
+
+afterEach(() => {
+  vi.unstubAllGlobals();
+});
+
+describe("Quick Chat persistent close/reopen lifecycle", () => {
+  it("does not mount ChatView before Quick Chat has ever opened", () => {
+    render(<QuickChatHarness projectId="project-a" onChatMount={() => {}} />);
+
+    expect(screen.queryByTestId("quick-chat-body")).toBeNull();
+  });
+
+  it("retains the same chat instance, session, and scroll position across close and reopen", async () => {
+    const onChatMount = vi.fn();
+    render(<QuickChatHarness projectId="project-a" onChatMount={onChatMount} />);
+
+    fireEvent.click(screen.getByRole("button", { name: "Open Quick Chat" }));
+    const chatBody = await screen.findByTestId("quick-chat-body");
+    const scroll = screen.getByTestId("quick-chat-scroll");
+    Object.defineProperty(scroll, "scrollTop", { configurable: true, value: 143, writable: true });
+    fireEvent.click(screen.getByRole("button", { name: "Change session" }));
+    expect(screen.getByTestId("quick-chat-session")).toHaveTextContent("Session two");
+    expect(onChatMount).toHaveBeenCalledTimes(1);
+
+    fireEvent.click(screen.getByRole("button", { name: "Close Quick Chat" }));
+    expect(screen.getByTestId("quick-chat-body")).toBe(chatBody);
+    expect(screen.getByTestId("floating-window-overlay-quick-chat-persist")).toHaveClass("floating-window-overlay--hidden");
+
+    fireEvent.click(screen.getByRole("button", { name: "Open Quick Chat" }));
+    expect(screen.getByTestId("quick-chat-body")).toBe(chatBody);
+    expect(screen.getByTestId("quick-chat-scroll")).toHaveProperty("scrollTop", 143);
+    expect(screen.getByTestId("quick-chat-session")).toHaveTextContent("Session two");
+    expect(onChatMount).toHaveBeenCalledTimes(1);
+  });
+
+  it("hides the retained desktop window and the mobile full-screen sheet", async () => {
+    vi.stubGlobal("matchMedia", vi.fn((query: string) => ({
+      matches: query === "(max-width: 768px)",
+      media: query,
+      onchange: null,
+      addEventListener: vi.fn(),
+      removeEventListener: vi.fn(),
+    })));
+    render(<QuickChatHarness projectId="project-a" onChatMount={() => {}} />);
+
+    fireEvent.click(screen.getByRole("button", { name: "Open Quick Chat" }));
+    await screen.findByTestId("quick-chat-body");
+    fireEvent.click(screen.getByRole("button", { name: "Close Quick Chat" }));
+
+    const overlay = screen.getByTestId("floating-window-overlay-quick-chat-persist");
+    expect(overlay).toHaveClass("floating-window-overlay--hidden");
+    expect(screen.getByTestId("quick-chat-body")).toBeTruthy();
+  });
+
+  it("unmounts the old project's retained chat instead of leaking it into the next project", async () => {
+    const onChatMount = vi.fn();
+    const { rerender } = render(<QuickChatHarness projectId="project-a" onChatMount={onChatMount} />);
+    fireEvent.click(screen.getByRole("button", { name: "Open Quick Chat" }));
+    await screen.findByTestId("quick-chat-body");
+    expect(screen.getByTestId("quick-chat-body")).toHaveAttribute("data-project-id", "project-a");
+
+    rerender(<QuickChatHarness projectId="project-b" onChatMount={onChatMount} />);
+    await waitFor(() => expect(screen.getByTestId("quick-chat-body")).toHaveAttribute("data-project-id", "project-b"));
+    expect(onChatMount).toHaveBeenCalledTimes(2);
+  });
+});