matrix: Fix flaky host mode tests

packages/matrix/helpers/index.ts

@@ -165,13 +165,49 @@ export async function createRealm(
   endpoint: string,
   name = endpoint,
 ) {
-  await page.locator('[data-test-add-workspace]').click();
-  await page.locator('[data-test-display-name-field]').fill(name);
-  await page.locator('[data-test-endpoint-field]').fill(endpoint);
-  await page.locator('[data-test-create-workspace-submit]').click();
-  await expect(page.locator(`[data-test-workspace="${name}"]`)).toHaveCount(1, {
-    timeout: 30_000,
-  });
+  // Creating a workspace provisions a matrix room + personal realm. Under
+  // load that can transiently fail: the modal surfaces an error
+  // (`data-test-error-message`) and stays open instead of closing and
+  // rendering the workspace tile. Submitting and waiting only for the tile
+  // then burns the full timeout on a modal that will never resolve. Instead
+  // wait for whichever outcome happens first and, on a surfaced error, retry
+  // the whole form from a clean modal (cancel re-opens with error cleared).
+  let workspaceTile = page.locator(`[data-test-workspace="${name}"]`);
+  let errorMessage = page.locator('[data-test-error-message]');
+  let maxAttempts = 3;
+  for (let attempt = 1; attempt <= maxAttempts; attempt++) {
+    // A previous attempt's provisioning may have landed late — if the tile is
+    // already present, don't try to create a duplicate endpoint.
+    if ((await workspaceTile.count()) > 0) {
+      return;
+    }
+
+    await page.locator('[data-test-add-workspace]').click();
+    await page.locator('[data-test-display-name-field]').fill(name);
+    await page.locator('[data-test-endpoint-field]').fill(endpoint);
+    await page.locator('[data-test-create-workspace-submit]').click();
+
+    await expect(workspaceTile.or(errorMessage).first()).toBeVisible({
+      timeout: 30_000,
+    });
+    if ((await workspaceTile.count()) > 0) {
+      return;
+    }
+
+    let message = (await errorMessage.textContent())?.trim() ?? '(no message)';
+    if (attempt === maxAttempts) {
+      throw new Error(
+        `createRealm("${endpoint}") failed after ${maxAttempts} attempts: ${message}`,
+      );
+    }
+    console.log(
+      `[createRealm] "${endpoint}" attempt ${attempt}/${maxAttempts} errored, retrying: ${message}`,
+    );
+    // Dismiss the errored modal so the retry opens a fresh one (which clears
+    // the error and resets the fields).
+    await page.locator('[data-test-cancel-create-workspace]').click();
+    await expect(errorMessage).toBeHidden();
+  }
 }
 
 export async function openRoot(page: Page, url = testHost) {

packages/matrix/helpers/isolated-realm-server.ts

@@ -277,6 +277,16 @@ export async function startPrerenderServer(
     BOXEL_HOST_URL: 'https://localhost:4200',
     LOG_LEVELS:
       process.env.TEST_HARNESS_PRERENDER_LOG_LEVELS ?? process.env.LOG_LEVELS,
+    // One prerender server is shared by both Playwright workers
+    // (fullyParallel) for the whole shard. With the pool size unset it
+    // collapses to a fixed 4 tabs, which the shard's concurrent publish +
+    // index work can exhaust — the pool thrashes (`standby refill failed
+    // to produce a fresh tab`, cross-affinity steals) and realm-server
+    // requests stall, surfacing as 60s page.goto / _publish-realm
+    // timeouts. Enable the dynamic envelope: keep a 4-tab idle floor (no
+    // extra baseline memory) but let it burst to 8 under load.
+    PRERENDER_PAGE_POOL_MIN: process.env.PRERENDER_PAGE_POOL_MIN ?? '4',
+    PRERENDER_PAGE_POOL_MAX: process.env.PRERENDER_PAGE_POOL_MAX ?? '8',
   };
   let prerenderArgs = [
     '--transpileOnly',