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.tsx

@@ -102,6 +102,17 @@ interface TerminalViewProps {
    * the mobile key toolbar. Not called when xterm fails to mount (headless).
    */
   onReady?: (api: TerminalInputApi) => void;
+  /**
+   * Bumped by the caller after a structural layout mutation (split/merge,
+   * ticket #61) so surviving terminals refit even when their container's
+   * `ResizeObserver` doesn't fire a useful event for the mutation (e.g. a
+   * sibling appearing/disappearing without this cell's own box changing size
+   * in a way the observer flags in time). Any value change schedules exactly
+   * one coalesced refit on the next animation frame, reusing the same
+   * zero-size guard and rows/cols-changed check as the resize-observer path —
+   * it never remounts/reopens the terminal.
+   */
+  refitSignal?: number;
 }
 
 /**
@@ -132,6 +143,7 @@ export function TerminalView({
   agentMode = false,
   portal,
   onReady,
+  refitSignal,
 }: TerminalViewProps) {
   const { terminal } = useGateways();
   const containerRef = useRef<HTMLDivElement | null>(null);
@@ -165,6 +177,10 @@ export function TerminalView({
   portalRef.current = portal;
   const onReadyRef = useRef(onReady);
   onReadyRef.current = onReady;
+  // Holds the mounted instance's `refit` closure so the `refitSignal` effect
+  // below (a separate effect, since it must NOT re-run/reopen the terminal on
+  // every parent render) can trigger it without depending on `cwd`'s effect.
+  const refitRef = useRef<(() => void) | null>(null);
 
   useEffect(() => {
     const container = containerRef.current;
@@ -337,14 +353,21 @@ export function TerminalView({
         void handle.resize(term.rows, term.cols);
       }
     };
-    const ro = new ResizeObserver(() => {
+    const scheduleRefit = () => {
       if (rafId) cancelAnimationFrame(rafId);
       rafId = requestAnimationFrame(refit);
-    });
+    };
+    const ro = new ResizeObserver(scheduleRefit);
     ro.observe(container);
+    // Let the `refitSignal` effect below trigger the SAME coalesced refit after
+    // a structural layout mutation (split/merge, ticket #61) — surviving cells
+    // don't always get a timely useful ResizeObserver event from a sibling
+    // appearing/disappearing.
+    refitRef.current = scheduleRefit;
 
     return () => {
       disposed = true;
+      refitRef.current = null;
       if (rafId) cancelAnimationFrame(rafId);
       ro.disconnect();
       onKey.dispose();
@@ -360,6 +383,20 @@ export function TerminalView({
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [cwd]);
 
+  // Ticket #61 — explicit refit after a structural layout mutation (split /
+  // merge). A survivor's own container may not fire a timely, useful
+  // ResizeObserver event when a SIBLING cell appears/disappears, so the caller
+  // (LayoutGrid, via `useLayout`'s layout-tree version) bumps `refitSignal` on
+  // every successful mutation and we reuse the mounted instance's coalesced
+  // `refit` (same zero-size guard, same rows/cols-changed check) instead of
+  // remounting/reopening the terminal. Deliberately a SEPARATE effect from the
+  // `[cwd]` one above so bumping the signal never re-runs the open/reattach
+  // logic.
+  useEffect(() => {
+    if (refitSignal === undefined) return;
+    refitRef.current?.();
+  }, [refitSignal]);
+
   return (
     <div
       data-testid="terminal-view"