diff --git a/playwright/e2e/crypto/crypto.spec.ts b/playwright/e2e/crypto/crypto.spec.ts
index f3a4820ebe..e520d971ea 100644
--- a/playwright/e2e/crypto/crypto.spec.ts
+++ b/playwright/e2e/crypto/crypto.spec.ts
@@ -43,6 +43,7 @@ const testMessages = async (page: Page, bob: Bot, bobRoomId: string) => {
 };
 
 const bobJoin = async (page: Page, bob: Bot) => {
+    // Wait for Bob to get the invite
     await bob.evaluate(async (cli) => {
         const bobRooms = cli.getRooms();
         if (!bobRooms.length) {
@@ -55,9 +56,13 @@ const bobJoin = async (page: Page, bob: Bot) => {
             });
         }
     });
-    const roomId = await bob.joinRoomByName("Alice");
 
+    const roomId = await bob.joinRoomByName("Alice");
     await expect(page.getByText("Bob joined the room")).toBeVisible();
+
+    // Even though Alice has seen Bob's join event, Bob may not have done so yet. Wait for the sync to arrive.
+    await bob.awaitRoomMembership(roomId);
+
     return roomId;
 };
 
diff --git a/playwright/e2e/crypto/event-shields.spec.ts b/playwright/e2e/crypto/event-shields.spec.ts
index fa9d1959da..0544a7c904 100644
--- a/playwright/e2e/crypto/event-shields.spec.ts
+++ b/playwright/e2e/crypto/event-shields.spec.ts
@@ -33,7 +33,7 @@ test.describe("Cryptography", function () {
             await app.client.bootstrapCrossSigning(aliceCredentials);
             await autoJoin(bob);
 
-            // create an encrypted room
+            // create an encrypted room, and wait for Bob to join it.
             testRoomId = await createSharedRoomWithUser(app, bob.credentials.userId, {
                 name: "TestRoom",
                 initial_state: [
@@ -46,6 +46,9 @@ test.describe("Cryptography", function () {
                     },
                 ],
             });
+
+            // Even though Alice has seen Bob's join event, Bob may not have done so yet. Wait for the sync to arrive.
+            await bob.awaitRoomMembership(testRoomId);
         });
 
         test("should show the correct shield on e2e events", async ({
@@ -287,9 +290,9 @@ test.describe("Cryptography", function () {
             // Let our app start syncing again
             await app.client.network.goOnline();
 
-            // Wait for the messages to arrive
+            // Wait for the messages to arrive. It can take quite a while for the sync to wake up.
             const last = page.locator(".mx_EventTile_last");
-            await expect(last).toContainText("test encrypted from unverified");
+            await expect(last).toContainText("test encrypted from unverified", { timeout: 20000 });
             const lastE2eIcon = last.locator(".mx_EventTile_e2eIcon");
             await expect(lastE2eIcon).toHaveClass(/mx_EventTile_e2eIcon_warning/);
             await lastE2eIcon.focus();
diff --git a/playwright/e2e/utils.ts b/playwright/e2e/utils.ts
index b357b5ca99..a2bcc0f29a 100644
--- a/playwright/e2e/utils.ts
+++ b/playwright/e2e/utils.ts
@@ -20,6 +20,14 @@ import { Client } from "../pages/client";
  * @param client Client instance that can be user or bot
  * @param roomId room id to find room and check
  * @param predicate defines condition that is used to check the room state
+ *
+ * FIXME this does not do what it is supposed to do, and I think it is unfixable.
+ *   `page.exposeFunction` adds a function which returns a Promise. `window[predicateId](room)` therefore
+ *   always returns a truthy value (a Promise).  But even if you fix that: as far as I can tell, the Room is
+ *   just passed to the callback function as a JSON blob: you cannot actually call any methods on it, so the
+ *   callback is useless.
+ *
+ * @deprecated This function is broken.
  */
 export async function waitForRoom(
     page: Page,
diff --git a/playwright/pages/client.ts b/playwright/pages/client.ts
index 06e05fdcfa..7d62f42ae0 100644
--- a/playwright/pages/client.ts
+++ b/playwright/pages/client.ts
@@ -289,6 +289,54 @@ export class Client {
         await client.evaluate((client, { roomId, userId }) => client.unban(roomId, userId), { roomId, userId });
     }
 
+    /**
+     * Wait for the client to have specific membership of a given room
+     *
+     * This is often useful after joining a room, when we need to wait for the sync loop to catch up.
+     *
+     * Times out with an error after 1 second.
+     *
+     * @param roomId - ID of the room to check
+     * @param membership - required membership.
+     */
+    public async awaitRoomMembership(roomId: string, membership: string = "join") {
+        await this.evaluate(
+            (cli: MatrixClient, { roomId, membership }) => {
+                const isReady = () => {
+                    // Fetch the room on each check, because we get a different instance before and after the join arrives.
+                    const room = cli.getRoom(roomId);
+                    const myMembership = room?.getMyMembership();
+                    // @ts-ignore access to private field "logger"
+                    cli.logger.info(`waiting for room ${roomId}: membership now ${myMembership}`);
+                    return myMembership === membership;
+                };
+                if (isReady()) return;
+
+                const timeoutPromise = new Promise((resolve) => setTimeout(resolve, 1000)).then(() => {
+                    const room = cli.getRoom(roomId);
+                    const myMembership = room?.getMyMembership();
+                    throw new Error(
+                        `Timeout waiting for room ${roomId} membership (now '${myMembership}', wanted '${membership}')`,
+                    );
+                });
+
+                const readyPromise = new Promise<void>((resolve) => {
+                    async function onEvent() {
+                        if (isReady()) {
+                            cli.removeListener(window.matrixcs.ClientEvent.Event, onEvent);
+                            resolve();
+                        }
+                    }
+
+                    cli.on(window.matrixcs.ClientEvent.Event, onEvent);
+                });
+
+                return Promise.race([timeoutPromise, readyPromise]);
+            },
+            { roomId, membership },
+        );
+    }
+
     /**
      * @param {MatrixEvent} event
      * @param {ReceiptType} receiptType