fix(frontend): refit différé des cellules terminal après mutation de layout (#61)

Le ResizeObserver de TerminalView ne déclenche pas toujours un événement
utile quand une cellule voisine apparaît/disparaît (split/merge), forçant
l'utilisateur à redimensionner la fenêtre pour rafraîchir le scaling xterm.
useLayout expose un layoutVersion bumpé à chaque commit d'arbre (chargement
initial inclus), relayé par LayoutGrid comme refitSignal à chaque
TerminalView survivant pour déclencher fit.fit() sans rouvrir le PTY.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/features/terminals/TerminalView.test.tsx

@@ -13,6 +13,8 @@
 import { describe, it, expect, vi, beforeAll, afterAll } from "vitest";
 import { render, screen, waitFor } from "@testing-library/react";
 
+import { FitAddon } from "@xterm/addon-fit";
+
 import type {
   Gateways,
   ReattachResult,
@@ -289,4 +291,67 @@ describe("TerminalView — visible launch-failure surface (ticket #14 F3)", () =
     expect(screen.queryByRole("alert")).toBeNull();
     expect(screen.queryByTestId("terminal-error")).toBeNull();
   });
+
+  describe("refitSignal (ticket #61 — refit after split/merge)", () => {
+    it("refits WITHOUT reopening the terminal when refitSignal changes", async () => {
+      // Simulates LayoutGrid bumping `useLayout`'s layout version after a
+      // split/merge: a survivor cell must refit its xterm grid, but must NOT
+      // re-run open/reattach — that would tear down and relaunch its PTY.
+      const fitSpy = vi.spyOn(FitAddon.prototype, "fit");
+      const open = vi.fn(async () => makeHandle({ sessionId: "survivor-1" }));
+
+      const { rerender } = renderView(new MockTerminalGateway(), "/cwd", {
+        open,
+        refitSignal: 1,
+      });
+      await waitFor(() => expect(open).toHaveBeenCalledTimes(1));
+
+      const fitCallsAtMount = fitSpy.mock.calls.length;
+      expect(fitCallsAtMount).toBeGreaterThan(0);
+
+      // jsdom reports a zero-size layout box, which the coalesced refit
+      // deliberately skips (the same guard that protects the resize-observer
+      // path from fitting to a transient zero size). Stub a real size on the
+      // inner xterm container so the refit triggered below actually reaches
+      // `fit.fit()` instead of bailing on the zero-size guard.
+      const container = screen.getByTestId("terminal-view").firstElementChild as HTMLElement;
+      Object.defineProperty(container, "clientWidth", { value: 400, configurable: true });
+      Object.defineProperty(container, "clientHeight", { value: 200, configurable: true });
+
+      rerender(
+        <DIProvider gateways={{ terminal: new MockTerminalGateway() } as unknown as Gateways}>
+          <TerminalView cwd="/cwd" open={open} refitSignal={2} />
+        </DIProvider>,
+      );
+
+      await waitFor(() =>
+        expect(fitSpy.mock.calls.length).toBeGreaterThan(fitCallsAtMount),
+      );
+      // The structural-mutation refit must never reopen the PTY.
+      expect(open).toHaveBeenCalledTimes(1);
+
+      fitSpy.mockRestore();
+    });
+
+    it("does not refit when refitSignal is left undefined (no-op for callers that don't pass it)", async () => {
+      const fitSpy = vi.spyOn(FitAddon.prototype, "fit");
+      const open = vi.fn(async () => makeHandle({ sessionId: "no-signal-1" }));
+
+      const { rerender } = renderView(new MockTerminalGateway(), "/cwd", { open });
+      await waitFor(() => expect(open).toHaveBeenCalledTimes(1));
+      fitSpy.mockClear();
+
+      rerender(
+        <DIProvider gateways={{ terminal: new MockTerminalGateway() } as unknown as Gateways}>
+          <TerminalView cwd="/cwd" open={open} />
+        </DIProvider>,
+      );
+
+      // Give any stray rAF a chance to fire, then assert nothing extra happened.
+      await new Promise((resolve) => requestAnimationFrame(resolve));
+      expect(fitSpy).not.toHaveBeenCalled();
+
+      fitSpy.mockRestore();
+    });
+  });
 });