fix(app-tauri,frontend): le panneau MCP se détache en fenêtre OS — #299 (cause: enum Rust ViewPanel sans variante Mcp, open_view_window("mcp") rejetait INVALID et detachPanel ne touchait le placement qu'en cas de succès donc aucun effet visible ni message; fix: variante ViewPanel::Mcp avec parse/as_str/title "MCP" + contrat stable label view-mcp / url index.html?panel=mcp / decodage view_panel_from_window_label, identite persistee des fenetres view-mcp acceptee a la restauration #50, et ProjectsView surface desormais un echec de detachement via alerte role=alert avec bouton Reessayer au lieu de l'echec silencieux — placement conserve tant que la fenetre OS ne s'ouvre pas; QA verte reconfirmee par Git: rustup run stable cargo fmt --all -- --check, git diff --check, cargo test -p app-tauri view_window_tests 9/9 dont mcp_view_window_contract_is_stable, persisted_identity_accepts_mcp_view_label 1/1, npm test 22/22 sur ViewWindow/ProjectsView.detach/viewPlacement, npm run build tsc+vite OK; fichiers tickets/296/*, tickets/299/, scripts/ et sync counter/index hors perimetre; aucun push)

frontend/src/app/ViewWindow.test.tsx

@@ -44,6 +44,13 @@ describe("parseViewWindowParams (#47)", () => {
     });
   });
 
+  it("accepts the detachable MCP panel", () => {
+    expect(parseViewWindowParams("?panel=mcp")).toEqual({
+      kind: "panel",
+      panel: "mcp",
+    });
+  });
+
   it("parses a plugin layout window query from the backend base64url contract", () => {
     expect(
       parseViewWindowParams(

frontend/src/features/projects/ProjectsView.detach.test.tsx

@@ -5,7 +5,7 @@
  * main window), and the backend's close event re-toggles it back out of
  * "detached". The one-slot invariant holds throughout.
  */
-import { describe, it, expect } from "vitest";
+import { describe, it, expect, vi } from "vitest";
 import { render, screen, within, waitFor, fireEvent } from "@testing-library/react";
 
 import {
@@ -83,6 +83,41 @@ describe("ProjectsView detach to OS window (#23)", () => {
     expect(screen.queryByRole("dialog", { name: "Git" })).toBeNull();
   });
 
+  it("detaches the MCP panel through the same OS-window path", async () => {
+    const { windowGateway } = await renderWithProject();
+
+    pickPlacement("MCP", "Fenêtre détachée");
+
+    await waitFor(() => expect(windowGateway.open.has("mcp")).toBe(true));
+    expect(screen.queryByRole("dialog", { name: "MCP" })).toBeNull();
+  });
+
+  it("keeps the panel in place and surfaces a retry when detaching fails", async () => {
+    const { windowGateway } = await renderWithProject();
+    const openViewWindow = vi
+      .spyOn(windowGateway, "openViewWindow")
+      .mockRejectedValueOnce(new Error("INVALID"))
+      .mockResolvedValue(undefined);
+
+    pickPlacement("MCP", "Flottant");
+    await screen.findByRole("dialog", { name: "MCP" });
+    fireEvent.click(
+      screen.getByRole("button", { name: "Fenêtre détachée — MCP" }),
+    );
+
+    const alert = await screen.findByRole("alert");
+    expect(alert.textContent).toContain(
+      "Le panneau MCP ne peut pas être détaché pour le moment.",
+    );
+    expect(screen.getByRole("dialog", { name: "MCP" })).toBeTruthy();
+
+    fireEvent.click(within(alert).getByRole("button", { name: "Réessayer" }));
+    await waitFor(() => expect(openViewWindow).toHaveBeenCalledTimes(2));
+    await waitFor(() =>
+      expect(screen.queryByRole("dialog", { name: "MCP" })).toBeNull(),
+    );
+  });
+
   it("re-toggles the placement out of detached when the OS window closes", async () => {
     const { windowGateway } = await renderWithProject();
 

frontend/src/features/projects/ProjectsView.tsx

@@ -246,6 +246,7 @@ export function ProjectsView() {
   const [pendingConversationOpen, setPendingConversationOpen] =
     useState<PendingConversationOpen | null>(null);
   const [taskToasts, setTaskToasts] = useState<BackgroundTaskToast[]>([]);
+  const [detachErrorPanel, setDetachErrorPanel] = useState<PanelId | null>(null);
 
   const active = vm.openTabs.find((t) => t.id === vm.activeTabId) ?? null;
   const activeViewerConversation =
@@ -446,11 +447,17 @@ export function ProjectsView() {
   // shell. On failure the placement is left untouched (no ghost "detached" slot).
   function detachPanel(panel: PanelId) {
     if (!active) return;
+    setDetachErrorPanel(null);
     void windowGateway
       .openViewWindow(panel)
-      .then(() => setPlacement(panel, "detached"))
+      .then(() => {
+        setDetachErrorPanel(null);
+        setPlacement(panel, "detached");
+      })
       .catch(() => {
-        /* window failed to open; keep the current placement */
+        // Keep the current placement (no ghost detached slot), but surface the
+        // failure so the action never appears to do nothing (#299).
+        setDetachErrorPanel(panel);
       });
   }
 
@@ -868,6 +875,20 @@ export function ProjectsView() {
           {vm.error}
         </p>
       )}
+      {detachErrorPanel && (
+        <div
+          role="alert"
+          className="mx-4 mt-2 flex shrink-0 items-center justify-between gap-3 rounded-md border border-danger/40 bg-danger/10 px-3 py-2 text-sm text-danger"
+        >
+          <span>
+            Le panneau {PANEL_TITLE[detachErrorPanel]} ne peut pas être détaché
+            pour le moment.
+          </span>
+          <Button size="sm" onClick={() => detachPanel(detachErrorPanel)}>
+            Réessayer
+          </Button>
+        </div>
+      )}
 
       {/* ── Project tab bar (#26): tabs = open projects, "+" opens Projects ── */}
       <ProjectTabs

frontend/src/features/projects/viewPlacement.test.ts

@@ -24,6 +24,7 @@ describe("isDetachablePanel (#50)", () => {
     expect(isDetachablePanel("git")).toBe(true);
     expect(isDetachablePanel("tickets")).toBe(true);
     expect(isDetachablePanel("memory")).toBe(true);
+    expect(isDetachablePanel("mcp")).toBe(true);
   });
 
   it("excludes projects (main-window-only chrome) and unknown panels", () => {