fix: defer xterm fit to ResizeObserver ticks

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

web/src/components/terminal/TerminalPanel.tsx

@@ -34,7 +34,6 @@ export function TerminalPanel() {
       terminal.writeln("$ pnpm test");
       terminal.writeln("mock: 3 files checked, 0 failures");
       terminal.write("$ ");
-      fitAddon.fit();
     } catch {
       setFailed(true);
       terminal?.dispose();
@@ -42,13 +41,34 @@ export function TerminalPanel() {
       return;
     }
 
+    let rafId: number | undefined;
+    const scheduleFit = () => {
+      if (rafId !== undefined) return;
+      rafId = requestAnimationFrame(() => {
+        rafId = undefined;
+        try {
+          fitAddon?.fit();
+        } catch {
+          // fit() throws when the container has zero dimensions; ignore
+          // and wait for the next ResizeObserver tick.
+        }
+      });
+    };
+
+    const resizeObserver = new ResizeObserver(scheduleFit);
+    resizeObserver.observe(container);
+
     const handleResize = () => {
-      fitAddon?.fit();
+      scheduleFit();
     };
 
     window.addEventListener("resize", handleResize);
 
     return () => {
+      if (rafId !== undefined) {
+        cancelAnimationFrame(rafId);
+      }
+      resizeObserver.disconnect();
       window.removeEventListener("resize", handleResize);
       terminal?.dispose();
       fitAddon?.dispose();