feat: F4.6g1 the echo server and the generic client windows

Oracle EchoWindowRun uses Java's echo server and client windows as a
person does; SaveAfterRunRun echo saves what Java keeps after the run.
The room takes setActive with a port, connect/disconnect/send, sends
both views and writes the server's port and the client's target back.
The editor's windows show Java's buttons, fields and lines; a port that
is no port is marked and not handed over (Java's button throws).
Core fix: an empty message (one empty segment with the push, as Java's
erstelleSegmente makes) counted as a repeat and never arrived.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/editor/browser/echo.spec.ts

@@ -0,0 +1,86 @@
+// The echo server and the generic client on two computers' desktops in
+// Chromium (F4.6g1), on browser/fixtures/web-pages.fls: installed while the
+// simulation runs; the server started on a port typed (one that is no port
+// marked and not handed over), the client connected, a message sent and
+// echoed, the fields locked while connected, the server's lines, "Trennen".
+
+import { dirname, join } from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { expect, test, type Page } from '@playwright/test';
+import { drawing, onDevice, serveEditor } from './editorPage.js';
+
+let server: { readonly base: string; readonly close: () => Promise<void> };
+
+test.beforeAll(async () => {
+  server = await serveEditor();
+});
+
+test.afterAll(async () => {
+  await server.close();
+});
+
+const FIXTURE = join(dirname(fileURLToPath(import.meta.url)), 'fixtures/web-pages.fls');
+
+async function openPages(page: Page): Promise<void> {
+  await page.goto(`${server.base}?example=new`);
+  await page.waitForSelector('svg.filius-canvas');
+  await page.locator('input[type="file"]').first().setInputFiles(FIXTURE);
+  await expect.poll(async () => (await drawing(page).catch(() => ({ nodes: [] }))).nodes.length).toBe(4);
+  await page.getByRole('button', { name: 'Wechselt in den Aktionsmodus' }).click();
+  await expect(page.locator('svg.filius-canvas')).toHaveAttribute('data-mode', 'simulation');
+}
+
+/** A computer's desktop opened with a tap, a program installed on it, and its window opened. */
+async function install(page: Page, name: string, program: string, label: string) {
+  const id = (await drawing(page)).nodes.find((node) => node.name === name)!.id;
+  const at = await onDevice(page, id);
+  await page.mouse.click(at.x, at.y);
+  const desktop = page.locator(`section[data-desktop="${id}"]`);
+  await desktop.locator('[data-program="installation"]').click();
+  const installation = desktop.locator('[data-program-window="installation"]');
+  await installation.locator('[data-list="available"] [role="option"]').filter({ hasText: label }).click();
+  await installation.getByRole('button', { name: 'Installieren' }).click();
+  await installation.getByRole('button', { name: 'Änderungen annehmen' }).click();
+  await desktop.locator(`[data-program="${program}"]`).click();
+  return { desktop, window: desktop.locator(`[data-program-window="${program}"]`) };
+}
+
+test('echoes a message from the generic client, as Java’s windows do', async ({ page }) => {
+  test.setTimeout(120_000);
+  await openPages(page);
+  const echo = await install(page, 'Server', 'echoServer', 'Echo-Server');
+  await expect(echo.window.locator('.program-titlebar h3')).toHaveText('Echo-Server');
+  const port = echo.window.getByRole('textbox', { name: 'Port:' });
+  await expect(port).toHaveValue('55555');
+  // A port that is no port: marked, the button does nothing.
+  await port.fill('abc');
+  await expect(port).toHaveAttribute('aria-invalid', 'true');
+  await echo.window.getByRole('button', { name: 'Starten' }).click();
+  await expect(echo.window.getByRole('button', { name: 'Starten' })).toBeVisible();
+  await port.fill('7777');
+  await echo.window.getByRole('button', { name: 'Starten' }).click();
+  await expect(echo.window.getByRole('button', { name: 'Anhalten' })).toBeVisible();
+  await expect(port).toHaveAttribute('readonly', '');
+  const serverLog = echo.window.getByLabel('Ausgabe');
+  await expect(serverLog).toHaveText('Annahme von Verbindungsanfragen gestartet\n');
+  await echo.desktop.locator('.desktop-close').click();
+
+  const client = await install(page, 'Browser', 'simpleClient', 'Einfacher Client');
+  const send = client.window.getByRole('button', { name: 'Senden' });
+  await expect(send).toBeDisabled();
+  await client.window.getByRole('textbox', { name: 'Server-Adresse:' }).fill('192.168.0.20');
+  await client.window.getByRole('textbox', { name: 'Server-Port:' }).fill('7777');
+  await client.window.getByRole('button', { name: 'Verbinden' }).click();
+  await expect(client.window.getByRole('button', { name: 'Trennen' })).toBeVisible({ timeout: 30_000 });
+  await expect(client.window.getByRole('textbox', { name: 'Server-Adresse:' })).toHaveAttribute('readonly', '');
+  const output = client.window.getByLabel('Ausgabe');
+  await expect(output).toHaveText('Verbindung hergestellt\n');
+  await client.window.getByRole('textbox', { name: 'Nachricht:' }).fill('Hallo Echo');
+  await send.click();
+  await expect(client.window.getByRole('textbox', { name: 'Nachricht:' })).toHaveValue('');
+  await expect(output).toHaveText('Verbindung hergestellt\n<<Hallo Echo\n>>Hallo Echo\n', { timeout: 30_000 });
+  await client.window.getByRole('button', { name: 'Trennen' }).click();
+  await expect(client.window.getByRole('button', { name: 'Verbinden' })).toBeVisible({ timeout: 30_000 });
+  await expect(output).toContainText('Verbindung getrennt');
+  await expect(send).toBeDisabled();
+});