diff --git a/.eslintrc.js b/.eslintrc.js
index 96ca83a..565a7de 100644
--- a/.eslintrc.js
+++ b/.eslintrc.js
@@ -8,6 +8,7 @@ module.exports = {
         browser: true,
     },
     rules: {
+        "no-duplicate-imports": ["error"],
         "no-var": ["warn"],
         "prefer-rest-params": ["warn"],
         "prefer-spread": ["warn"],
@@ -24,6 +25,8 @@ module.exports = {
                 asyncArrow: "always",
             },
         ],
+        "@typescript-eslint/await-thenable": ["error"],
+        "@typescript-eslint/prefer-readonly": ["error"],
         "arrow-parens": "off",
         "prefer-promise-reject-errors": "off",
         "quotes": "off",
diff --git a/examples/widget/utils.js b/examples/widget/utils.js
index 705a6f0..46eb78b 100644
--- a/examples/widget/utils.js
+++ b/examples/widget/utils.js
@@ -15,7 +15,7 @@
  */
 
 function parseFragment() {
-    const fragmentString = window.location.hash || "?";
+    const fragmentString = globalThis.location.hash || "?";
     return new URLSearchParams(fragmentString.substring(Math.max(fragmentString.indexOf("?"), 0)));
 }
 
diff --git a/src/ClientWidgetApi.ts b/src/ClientWidgetApi.ts
index 6404904..87f4cff 100644
--- a/src/ClientWidgetApi.ts
+++ b/src/ClientWidgetApi.ts
@@ -62,7 +62,6 @@ import {
 } from "./interfaces/SendToDeviceAction";
 import { EventDirection, EventKind, WidgetEventCapability } from "./models/WidgetEventCapability";
 import { IRoomEvent } from "./interfaces/IRoomEvent";
-import { IRoomAccountData } from "./interfaces/IRoomAccountData";
 import {
     IGetOpenIDActionRequest,
     IGetOpenIDActionResponseData,
@@ -141,15 +140,15 @@ export class ClientWidgetApi extends EventEmitter {
     private cachedWidgetVersions: ApiVersion[] | null = null;
     // contentLoadedActionSent is used to check that only one ContentLoaded request is send.
     private contentLoadedActionSent = false;
-    private allowedCapabilities = new Set<Capability>();
-    private allowedEvents: WidgetEventCapability[] = [];
+    private readonly allowedCapabilities = new Set<Capability>();
+    private readonly allowedEvents: WidgetEventCapability[] = [];
     private isStopped = false;
     private turnServers: AsyncGenerator<ITurnServer> | null = null;
     private contentLoadedWaitTimer?: ReturnType<typeof setTimeout>;
     // Stores pending requests to push a room's state to the widget
-    private pushRoomStateTasks = new Set<Promise<void>>();
+    private readonly pushRoomStateTasks = new Set<Promise<void>>();
     // Room ID → event type → state key → events to be pushed
-    private pushRoomStateResult = new Map<string, Map<string, Map<string, IRoomEvent>>>();
+    private readonly pushRoomStateResult = new Map<string, Map<string, Map<string, IRoomEvent>>>();
     private flushRoomStateTask: Promise<void> | null = null;
 
     /**
@@ -162,8 +161,8 @@ export class ClientWidgetApi extends EventEmitter {
      */
     public constructor(
         public readonly widget: Widget,
-        private iframe: HTMLIFrameElement,
-        private driver: WidgetDriver,
+        iframe: HTMLIFrameElement,
+        private readonly driver: WidgetDriver,
     ) {
         super();
         if (!iframe?.contentWindow) {
@@ -175,7 +174,12 @@ export class ClientWidgetApi extends EventEmitter {
         if (!driver) {
             throw new Error("Invalid driver");
         }
-        this.transport = new PostmessageTransport(WidgetApiDirection.ToWidget, widget.id, iframe.contentWindow, window);
+        this.transport = new PostmessageTransport(
+            WidgetApiDirection.ToWidget,
+            widget.id,
+            iframe.contentWindow,
+            globalThis,
+        );
         this.transport.targetOrigin = widget.origin;
         this.transport.on("message", this.handleMessage.bind(this));
 
@@ -230,7 +234,7 @@ export class ClientWidgetApi extends EventEmitter {
 
     public async getWidgetVersions(): Promise<ApiVersion[]> {
         if (Array.isArray(this.cachedWidgetVersions)) {
-            return Promise.resolve(this.cachedWidgetVersions);
+            return this.cachedWidgetVersions;
         }
 
         try {
@@ -381,7 +385,7 @@ export class ClientWidgetApi extends EventEmitter {
             });
         }
 
-        if (!request.data?.uri || !request.data?.uri.toString().startsWith("https://matrix.to/#")) {
+        if (!request.data?.uri.startsWith("https://matrix.to/#")) {
             return this.transport.reply<IWidgetApiErrorResponseData>(request, {
                 error: { message: "Invalid matrix.to URI" },
             });
@@ -467,9 +471,9 @@ export class ClientWidgetApi extends EventEmitter {
 
         this.driver.askOpenID(observer);
     }
+
     private handleReadRoomAccountData(request: IReadRoomAccountDataFromWidgetActionRequest): void | Promise<void> {
-        let events: Promise<IRoomAccountData[]> = Promise.resolve([]);
-        events = this.driver.readRoomAccountData(request.data.type);
+        const events = this.driver.readRoomAccountData(request.data.type);
 
         if (!this.canReceiveRoomAccountData(request.data.type)) {
             return this.transport.reply<IWidgetApiErrorResponseData>(request, {
@@ -609,17 +613,17 @@ export class ClientWidgetApi extends EventEmitter {
                 });
             }
 
-            if (!isDelayedEvent) {
-                sendEventPromise = this.driver.sendEvent(
+            if (isDelayedEvent) {
+                sendEventPromise = this.driver.sendDelayedEvent(
+                    request.data.delay ?? null,
+                    request.data.parent_delay_id ?? null,
                     request.data.type,
                     request.data.content || {},
                     request.data.state_key,
                     request.data.room_id,
                 );
             } else {
-                sendEventPromise = this.driver.sendDelayedEvent(
-                    request.data.delay ?? null,
-                    request.data.parent_delay_id ?? null,
+                sendEventPromise = this.driver.sendEvent(
                     request.data.type,
                     request.data.content || {},
                     request.data.state_key,
@@ -635,17 +639,17 @@ export class ClientWidgetApi extends EventEmitter {
                 });
             }
 
-            if (!isDelayedEvent) {
-                sendEventPromise = this.driver.sendEvent(
+            if (isDelayedEvent) {
+                sendEventPromise = this.driver.sendDelayedEvent(
+                    request.data.delay ?? null,
+                    request.data.parent_delay_id ?? null,
                     request.data.type,
                     content,
                     null, // not sending a state event
                     request.data.room_id,
                 );
             } else {
-                sendEventPromise = this.driver.sendDelayedEvent(
-                    request.data.delay ?? null,
-                    request.data.parent_delay_id ?? null,
+                sendEventPromise = this.driver.sendEvent(
                     request.data.type,
                     content,
                     null, // not sending a state event
@@ -715,25 +719,25 @@ export class ClientWidgetApi extends EventEmitter {
 
     private async handleSendToDevice(request: ISendToDeviceFromWidgetActionRequest): Promise<void> {
         if (!request.data.type) {
-            await this.transport.reply<IWidgetApiErrorResponseData>(request, {
+            this.transport.reply<IWidgetApiErrorResponseData>(request, {
                 error: { message: "Invalid request - missing event type" },
             });
         } else if (!request.data.messages) {
-            await this.transport.reply<IWidgetApiErrorResponseData>(request, {
+            this.transport.reply<IWidgetApiErrorResponseData>(request, {
                 error: { message: "Invalid request - missing event contents" },
             });
         } else if (typeof request.data.encrypted !== "boolean") {
-            await this.transport.reply<IWidgetApiErrorResponseData>(request, {
+            this.transport.reply<IWidgetApiErrorResponseData>(request, {
                 error: { message: "Invalid request - missing encryption flag" },
             });
         } else if (!this.canSendToDeviceEvent(request.data.type)) {
-            await this.transport.reply<IWidgetApiErrorResponseData>(request, {
+            this.transport.reply<IWidgetApiErrorResponseData>(request, {
                 error: { message: "Cannot send to-device events of this type" },
             });
         } else {
             try {
                 await this.driver.sendToDevice(request.data.type, request.data.encrypted, request.data.messages);
-                await this.transport.reply<ISendToDeviceFromWidgetResponseData>(request, {});
+                this.transport.reply<ISendToDeviceFromWidgetResponseData>(request, {});
             } catch (e) {
                 console.error("error sending to-device event", e);
                 this.handleDriverError(e, request, "Error sending event");
@@ -762,12 +766,12 @@ export class ClientWidgetApi extends EventEmitter {
 
     private async handleWatchTurnServers(request: IWatchTurnServersRequest): Promise<void> {
         if (!this.hasCapability(MatrixCapabilities.MSC3846TurnServers)) {
-            await this.transport.reply<IWidgetApiErrorResponseData>(request, {
+            this.transport.reply<IWidgetApiErrorResponseData>(request, {
                 error: { message: "Missing capability" },
             });
         } else if (this.turnServers) {
             // We're already polling, so this is a no-op
-            await this.transport.reply<IWidgetApiAcknowledgeResponseData>(request, {});
+            this.transport.reply<IWidgetApiAcknowledgeResponseData>(request, {});
         } else {
             try {
                 const turnServers = this.driver.getTurnServers();
@@ -776,14 +780,14 @@ export class ClientWidgetApi extends EventEmitter {
                 // client isn't banned from getting TURN servers entirely
                 const { done, value } = await turnServers.next();
                 if (done) throw new Error("Client refuses to provide any TURN servers");
-                await this.transport.reply<IWidgetApiAcknowledgeResponseData>(request, {});
+                this.transport.reply<IWidgetApiAcknowledgeResponseData>(request, {});
 
                 // Start the poll loop, sending the widget the initial result
                 this.pollTurnServers(turnServers, value);
                 this.turnServers = turnServers;
             } catch (e) {
                 console.error("error getting first TURN server results", e);
-                await this.transport.reply<IWidgetApiErrorResponseData>(request, {
+                this.transport.reply<IWidgetApiErrorResponseData>(request, {
                     error: { message: "TURN servers not available" },
                 });
             }
@@ -792,17 +796,17 @@ export class ClientWidgetApi extends EventEmitter {
 
     private async handleUnwatchTurnServers(request: IUnwatchTurnServersRequest): Promise<void> {
         if (!this.hasCapability(MatrixCapabilities.MSC3846TurnServers)) {
-            await this.transport.reply<IWidgetApiErrorResponseData>(request, {
+            this.transport.reply<IWidgetApiErrorResponseData>(request, {
                 error: { message: "Missing capability" },
             });
         } else if (!this.turnServers) {
             // We weren't polling anyways, so this is a no-op
-            await this.transport.reply<IWidgetApiAcknowledgeResponseData>(request, {});
+            this.transport.reply<IWidgetApiAcknowledgeResponseData>(request, {});
         } else {
             // Stop the generator, allowing it to clean up
             await this.turnServers.return(undefined);
             this.turnServers = null;
-            await this.transport.reply<IWidgetApiAcknowledgeResponseData>(request, {});
+            this.transport.reply<IWidgetApiAcknowledgeResponseData>(request, {});
         }
     }
 
@@ -1147,7 +1151,7 @@ export class ClientWidgetApi extends EventEmitter {
     private async flushRoomState(): Promise<void> {
         try {
             // Only send a single action once all concurrent tasks have completed
-            do await Promise.all([...this.pushRoomStateTasks]);
+            do await Promise.all(this.pushRoomStateTasks);
             while (this.pushRoomStateTasks.size > 0);
 
             const events: IRoomEvent[] = [];
@@ -1257,7 +1261,7 @@ export class ClientWidgetApi extends EventEmitter {
                     eventTypeMap.set(rawEvent.type, stateKeyMap);
                 }
                 if (!stateKeyMap.has(rawEvent.type)) stateKeyMap.set(rawEvent.state_key, rawEvent);
-                do await Promise.all([...this.pushRoomStateTasks]);
+                do await Promise.all(this.pushRoomStateTasks);
                 while (this.pushRoomStateTasks.size > 0);
                 await this.flushRoomStateTask;
             }
diff --git a/src/WidgetApi.ts b/src/WidgetApi.ts
index b35f826..73ee411 100644
--- a/src/WidgetApi.ts
+++ b/src/WidgetApi.ts
@@ -131,7 +131,7 @@ export class WidgetApi extends EventEmitter {
 
     private capabilitiesFinished = false;
     private supportsMSC2974Renegotiate = false;
-    private requestedCapabilities: Capability[] = [];
+    private readonly requestedCapabilities: Capability[] = [];
     private approvedCapabilities?: Capability[];
     private cachedClientVersions?: ApiVersion[];
     private turnServerWatchers = 0;
@@ -142,15 +142,17 @@ export class WidgetApi extends EventEmitter {
      * the API will use the widget ID from the first valid request it receives.
      * @param {string} clientOrigin The origin of the client, or null if not known.
      */
-    public constructor(
-        widgetId: string | null = null,
-        private clientOrigin: string | null = null,
-    ) {
+    public constructor(widgetId: string | null = null, clientOrigin: string | null = null) {
         super();
-        if (!window.parent) {
+        if (!globalThis.parent) {
             throw new Error("No parent window. This widget doesn't appear to be embedded properly.");
         }
-        this.transport = new PostmessageTransport(WidgetApiDirection.FromWidget, widgetId, window.parent, window);
+        this.transport = new PostmessageTransport(
+            WidgetApiDirection.FromWidget,
+            widgetId,
+            globalThis.parent,
+            globalThis,
+        );
         this.transport.targetOrigin = clientOrigin;
         this.transport.on("message", this.handleMessage.bind(this));
     }
@@ -191,7 +193,9 @@ export class WidgetApi extends EventEmitter {
      * @throws Throws if the capabilities negotiation has already started.
      */
     public requestCapabilities(capabilities: Capability[]): void {
-        capabilities.forEach((cap) => this.requestCapability(cap));
+        for (const cap of capabilities) {
+            this.requestCapability(cap);
+        }
     }
 
     /**
@@ -474,7 +478,7 @@ export class WidgetApi extends EventEmitter {
     }
 
     /**
-     * @deprecated This currently relies on an unstable MSC (MSC4157).
+     * @experimental This currently relies on an unstable MSC (MSC4157).
      */
     public cancelScheduledDelayedEvent(delayId: string): Promise<IUpdateDelayedEventFromWidgetResponseData> {
         return this.transport.send<IUpdateDelayedEventFromWidgetRequestData, IUpdateDelayedEventFromWidgetResponseData>(
@@ -487,7 +491,7 @@ export class WidgetApi extends EventEmitter {
     }
 
     /**
-     * @deprecated This currently relies on an unstable MSC (MSC4157).
+     * @experimental This currently relies on an unstable MSC (MSC4157).
      */
     public restartScheduledDelayedEvent(delayId: string): Promise<IUpdateDelayedEventFromWidgetResponseData> {
         return this.transport.send<IUpdateDelayedEventFromWidgetRequestData, IUpdateDelayedEventFromWidgetResponseData>(
@@ -500,7 +504,7 @@ export class WidgetApi extends EventEmitter {
     }
 
     /**
-     * @deprecated This currently relies on an unstable MSC (MSC4157).
+     * @experimental This currently relies on an unstable MSC (MSC4157).
      */
     public sendScheduledDelayedEvent(delayId: string): Promise<IUpdateDelayedEventFromWidgetResponseData> {
         return this.transport.send<IUpdateDelayedEventFromWidgetRequestData, IUpdateDelayedEventFromWidgetResponseData>(
@@ -681,7 +685,7 @@ export class WidgetApi extends EventEmitter {
      * @param {string} uri The URI to navigate to.
      * @returns {Promise<void>} Resolves when complete.
      * @throws Throws if the URI is invalid or cannot be processed.
-     * @deprecated This currently relies on an unstable MSC (MSC2931).
+     * @experimental This currently relies on an unstable MSC (MSC2931).
      */
     public navigateTo(uri: string): Promise<void> {
         if (!uri || !uri.startsWith("https://matrix.to/#")) {
@@ -704,7 +708,7 @@ export class WidgetApi extends EventEmitter {
         const onUpdateTurnServers = async (ev: CustomEvent<IUpdateTurnServersRequest>): Promise<void> => {
             ev.preventDefault();
             setTurnServer(ev.detail.data);
-            await this.transport.reply<IWidgetApiAcknowledgeResponseData>(ev.detail, {});
+            this.transport.reply<IWidgetApiAcknowledgeResponseData>(ev.detail, {});
         };
 
         // Start listening for updates before we even start watching, to catch
diff --git a/src/interfaces/Capabilities.ts b/src/interfaces/Capabilities.ts
index f541ac5..9b1ec15 100644
--- a/src/interfaces/Capabilities.ts
+++ b/src/interfaces/Capabilities.ts
@@ -26,28 +26,31 @@ export enum MatrixCapabilities {
      */
     RequiresClient = "io.element.requires_client",
     /**
-     * @deprecated It is not recommended to rely on this existing - it can be removed without notice.
+     * @experimental It is not recommended to rely on this existing - it can be removed without notice.
      */
     MSC2931Navigate = "org.matrix.msc2931.navigate",
+    /**
+     * @experimental It is not recommended to rely on this existing - it can be removed without notice.
+     */
     MSC3846TurnServers = "town.robin.msc3846.turn_servers",
     /**
-     * @deprecated It is not recommended to rely on this existing - it can be removed without notice.
+     * @experimental It is not recommended to rely on this existing - it can be removed without notice.
      */
     MSC3973UserDirectorySearch = "org.matrix.msc3973.user_directory_search",
     /**
-     * @deprecated It is not recommended to rely on this existing - it can be removed without notice.
+     * @experimental It is not recommended to rely on this existing - it can be removed without notice.
      */
     MSC4039UploadFile = "org.matrix.msc4039.upload_file",
     /**
-     * @deprecated It is not recommended to rely on this existing - it can be removed without notice.
+     * @experimental It is not recommended to rely on this existing - it can be removed without notice.
      */
     MSC4039DownloadFile = "org.matrix.msc4039.download_file",
     /**
-     * @deprecated It is not recommended to rely on this existing - it can be removed without notice.
+     * @experimental It is not recommended to rely on this existing - it can be removed without notice.
      */
     MSC4157SendDelayedEvent = "org.matrix.msc4157.send.delayed_event",
     /**
-     * @deprecated It is not recommended to rely on this existing - it can be removed without notice.
+     * @experimental It is not recommended to rely on this existing - it can be removed without notice.
      */
     MSC4157UpdateDelayedEvent = "org.matrix.msc4157.update_delayed_event",
 }
diff --git a/src/interfaces/WidgetApiAction.ts b/src/interfaces/WidgetApiAction.ts
index 2f0bcf5..b6ac75d 100644
--- a/src/interfaces/WidgetApiAction.ts
+++ b/src/interfaces/WidgetApiAction.ts
@@ -49,47 +49,47 @@ export enum WidgetApiFromWidgetAction {
     BeeperReadRoomAccountData = "com.beeper.read_room_account_data",
 
     /**
-     * @deprecated It is not recommended to rely on this existing - it can be removed without notice.
+     * @experimental It is not recommended to rely on this existing - it can be removed without notice.
      */
     MSC2876ReadEvents = "org.matrix.msc2876.read_events",
 
     /**
-     * @deprecated It is not recommended to rely on this existing - it can be removed without notice.
+     * @experimental It is not recommended to rely on this existing - it can be removed without notice.
      */
     MSC2931Navigate = "org.matrix.msc2931.navigate",
 
     /**
-     * @deprecated It is not recommended to rely on this existing - it can be removed without notice.
+     * @experimental It is not recommended to rely on this existing - it can be removed without notice.
      */
     MSC2974RenegotiateCapabilities = "org.matrix.msc2974.request_capabilities",
 
     /**
-     * @deprecated It is not recommended to rely on this existing - it can be removed without notice.
+     * @experimental It is not recommended to rely on this existing - it can be removed without notice.
      */
     MSC3869ReadRelations = "org.matrix.msc3869.read_relations",
 
     /**
-     * @deprecated It is not recommended to rely on this existing - it can be removed without notice.
+     * @experimental It is not recommended to rely on this existing - it can be removed without notice.
      */
     MSC3973UserDirectorySearch = "org.matrix.msc3973.user_directory_search",
 
     /**
-     * @deprecated It is not recommended to rely on this existing - it can be removed without notice.
+     * @experimental It is not recommended to rely on this existing - it can be removed without notice.
      */
     MSC4039GetMediaConfigAction = "org.matrix.msc4039.get_media_config",
 
     /**
-     * @deprecated It is not recommended to rely on this existing - it can be removed without notice.
+     * @experimental It is not recommended to rely on this existing - it can be removed without notice.
      */
     MSC4039UploadFileAction = "org.matrix.msc4039.upload_file",
 
     /**
-     * @deprecated It is not recommended to rely on this existing - it can be removed without notice.
+     * @experimental It is not recommended to rely on this existing - it can be removed without notice.
      */
     MSC4039DownloadFileAction = "org.matrix.msc4039.download_file",
 
     /**
-     * @deprecated It is not recommended to rely on this existing - it can be removed without notice.
+     * @experimental It is not recommended to rely on this existing - it can be removed without notice.
      */
     MSC4157UpdateDelayedEvent = "org.matrix.msc4157.update_delayed_event",
 }
diff --git a/src/models/Widget.ts b/src/models/Widget.ts
index 0b66452..0207b87 100644
--- a/src/models/Widget.ts
+++ b/src/models/Widget.ts
@@ -14,15 +14,14 @@
  * limitations under the License.
  */
 
-import { IWidget, IWidgetData, WidgetType } from "..";
+import { ITemplateParams, IWidget, IWidgetData, runTemplate, WidgetType } from "..";
 import { assertPresent } from "./validation/utils";
-import { ITemplateParams, runTemplate } from "..";
 
 /**
  * Represents the barest form of widget.
  */
 export class Widget {
-    public constructor(private definition: IWidget) {
+    public constructor(private readonly definition: IWidget) {
         if (!this.definition) throw new Error("Definition is required");
 
         assertPresent(definition, "id");
diff --git a/src/transport/PostmessageTransport.ts b/src/transport/PostmessageTransport.ts
index 4589735..b1c7125 100644
--- a/src/transport/PostmessageTransport.ts
+++ b/src/transport/PostmessageTransport.ts
@@ -45,9 +45,9 @@ export class PostmessageTransport extends EventEmitter implements ITransport {
     public timeoutSeconds = 10;
 
     private _ready = false;
-    private _widgetId: string | null = null;
-    private outboundRequests = new Map<string, IOutboundRequest | null>();
-    private stopController = new AbortController();
+    private _widgetId: string | null;
+    private readonly outboundRequests = new Map<string, IOutboundRequest | null>();
+    private readonly stopController = new AbortController();
 
     public get ready(): boolean {
         return this._ready;
@@ -58,10 +58,10 @@ export class PostmessageTransport extends EventEmitter implements ITransport {
     }
 
     public constructor(
-        private sendDirection: WidgetApiDirection,
-        private initialWidgetId: string | null,
-        private transportWindow: Window,
-        private inboundWindow: Window,
+        private readonly sendDirection: WidgetApiDirection,
+        initialWidgetId: string | null,
+        private readonly transportWindow: Window | typeof globalThis,
+        private readonly inboundWindow: Window | typeof globalThis,
     ) {
         super();
         this._widgetId = initialWidgetId;
@@ -159,21 +159,19 @@ export class PostmessageTransport extends EventEmitter implements ITransport {
         if (this.stopController.signal.aborted) return;
         if (!ev.data) return; // invalid event
 
-        if (this.strictOriginCheck && ev.origin !== window.origin) return; // bad origin
+        if (this.strictOriginCheck && ev.origin !== globalThis.origin) return; // bad origin
 
         // treat the message as a response first, then downgrade to a request
         const response = <IWidgetApiResponse>ev.data;
         if (!response.action || !response.requestId || !response.widgetId) return; // invalid request/response
 
-        if (!response.response) {
-            // it's a request
+        if (response.response) {
+            if (response.api !== this.sendDirection) return; // wrong direction
+            this.handleResponse(response);
+        } else {
             const request = <IWidgetApiRequest>response;
             if (request.api !== invertedDirection(this.sendDirection)) return; // wrong direction
             this.handleRequest(request);
-        } else {
-            // it's a response
-            if (response.api !== this.sendDirection) return; // wrong direction
-            this.handleResponse(response);
         }
     }
 
diff --git a/test/ClientWidgetApi-test.ts b/test/ClientWidgetApi-test.ts
index 1a49390..22c879a 100644
--- a/test/ClientWidgetApi-test.ts
+++ b/test/ClientWidgetApi-test.ts
@@ -214,12 +214,12 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: { message: "Missing capability" },
                 });
             });
 
-            expect(driver.navigate).not.toBeCalled();
+            expect(driver.navigate).not.toHaveBeenCalled();
         });
 
         it("fails to navigate to an unsupported URI", async () => {
@@ -238,12 +238,12 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: { message: "Invalid matrix.to URI" },
                 });
             });
 
-            expect(driver.navigate).not.toBeCalled();
+            expect(driver.navigate).not.toHaveBeenCalled();
         });
 
         it("should reject requests when the driver throws an exception", async () => {
@@ -264,7 +264,7 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: { message: "Error handling navigation" },
                 });
             });
@@ -294,7 +294,7 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: {
                         message: "Error handling navigation",
                         matrix_api_error: {
@@ -416,7 +416,7 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: { message: "Error sending event" },
                 });
             });
@@ -453,7 +453,7 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: {
                         message: "Error sending event",
                         matrix_api_error: {
@@ -498,112 +498,124 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: { message: expect.any(String) },
                 });
             });
 
-            expect(driver.sendDelayedEvent).not.toBeCalled();
+            expect(driver.sendDelayedEvent).not.toHaveBeenCalled();
         });
 
-        it("sends delayed message events", async () => {
-            const roomId = "!room:example.org";
-            const parentDelayId = "fp";
-            const timeoutDelayId = "ft";
+        it.each([
+            { hasDelay: true, hasParent: false },
+            { hasDelay: false, hasParent: true },
+            { hasDelay: true, hasParent: true },
+        ])(
+            "sends delayed message events (hasDelay = $hasDelay, hasParent = $hasParent)",
+            async ({ hasDelay, hasParent }) => {
+                const roomId = "!room:example.org";
+                const timeoutDelayId = "ft";
 
-            driver.sendDelayedEvent.mockResolvedValue({
-                roomId,
-                delayId: timeoutDelayId,
-            });
+                driver.sendDelayedEvent.mockResolvedValue({
+                    roomId,
+                    delayId: timeoutDelayId,
+                });
 
-            const event: ISendEventFromWidgetActionRequest = {
-                api: WidgetApiDirection.FromWidget,
-                widgetId: "test",
-                requestId: "0",
-                action: WidgetApiFromWidgetAction.SendEvent,
-                data: {
-                    type: "m.room.message",
-                    content: {},
-                    room_id: roomId,
-                    delay: 5000,
-                    parent_delay_id: parentDelayId,
-                },
-            };
+                const event: ISendEventFromWidgetActionRequest = {
+                    api: WidgetApiDirection.FromWidget,
+                    widgetId: "test",
+                    requestId: "0",
+                    action: WidgetApiFromWidgetAction.SendEvent,
+                    data: {
+                        type: "m.room.message",
+                        content: {},
+                        room_id: roomId,
+                        ...(hasDelay && { delay: 5000 }),
+                        ...(hasParent && { parent_delay_id: "fp" }),
+                    },
+                };
 
-            await loadIframe([
-                `org.matrix.msc2762.timeline:${event.data.room_id}`,
-                `org.matrix.msc2762.send.event:${event.data.type}`,
-                "org.matrix.msc4157.send.delayed_event",
-            ]);
+                await loadIframe([
+                    `org.matrix.msc2762.timeline:${event.data.room_id}`,
+                    `org.matrix.msc2762.send.event:${event.data.type}`,
+                    "org.matrix.msc4157.send.delayed_event",
+                ]);
 
-            emitEvent(new CustomEvent("", { detail: event }));
+                emitEvent(new CustomEvent("", { detail: event }));
 
-            await waitFor(() => {
-                expect(transport.reply).toHaveBeenCalledWith(event, {
-                    room_id: roomId,
-                    delay_id: timeoutDelayId,
+                await waitFor(() => {
+                    expect(transport.reply).toHaveBeenCalledWith(event, {
+                        room_id: roomId,
+                        delay_id: timeoutDelayId,
+                    });
                 });
-            });
 
-            expect(driver.sendDelayedEvent).toHaveBeenCalledWith(
-                event.data.delay,
-                event.data.parent_delay_id,
-                event.data.type,
-                event.data.content,
-                null,
-                roomId,
-            );
-        });
-
-        it("sends delayed state events", async () => {
-            const roomId = "!room:example.org";
-            const parentDelayId = "fp";
-            const timeoutDelayId = "ft";
+                expect(driver.sendDelayedEvent).toHaveBeenCalledWith(
+                    event.data.delay ?? null,
+                    event.data.parent_delay_id ?? null,
+                    event.data.type,
+                    event.data.content,
+                    null,
+                    roomId,
+                );
+            },
+        );
 
-            driver.sendDelayedEvent.mockResolvedValue({
-                roomId,
-                delayId: timeoutDelayId,
-            });
+        it.each([
+            { hasDelay: true, hasParent: false },
+            { hasDelay: false, hasParent: true },
+            { hasDelay: true, hasParent: true },
+        ])(
+            "sends delayed state events (hasDelay = $hasDelay, hasParent = $hasParent)",
+            async ({ hasDelay, hasParent }) => {
+                const roomId = "!room:example.org";
+                const timeoutDelayId = "ft";
+
+                driver.sendDelayedEvent.mockResolvedValue({
+                    roomId,
+                    delayId: timeoutDelayId,
+                });
 
-            const event: ISendEventFromWidgetActionRequest = {
-                api: WidgetApiDirection.FromWidget,
-                widgetId: "test",
-                requestId: "0",
-                action: WidgetApiFromWidgetAction.SendEvent,
-                data: {
-                    type: "m.room.topic",
-                    content: {},
-                    state_key: "",
-                    room_id: roomId,
-                    delay: 5000,
-                    parent_delay_id: parentDelayId,
-                },
-            };
+                const event: ISendEventFromWidgetActionRequest = {
+                    api: WidgetApiDirection.FromWidget,
+                    widgetId: "test",
+                    requestId: "0",
+                    action: WidgetApiFromWidgetAction.SendEvent,
+                    data: {
+                        type: "m.room.topic",
+                        content: {},
+                        state_key: "",
+                        room_id: roomId,
+                        ...(hasDelay && { delay: 5000 }),
+                        ...(hasParent && { parent_delay_id: "fp" }),
+                    },
+                };
 
-            await loadIframe([
-                `org.matrix.msc2762.timeline:${event.data.room_id}`,
-                `org.matrix.msc2762.send.state_event:${event.data.type}`,
-                "org.matrix.msc4157.send.delayed_event",
-            ]);
+                await loadIframe([
+                    `org.matrix.msc2762.timeline:${event.data.room_id}`,
+                    `org.matrix.msc2762.send.state_event:${event.data.type}`,
+                    "org.matrix.msc4157.send.delayed_event",
+                ]);
 
-            emitEvent(new CustomEvent("", { detail: event }));
+                emitEvent(new CustomEvent("", { detail: event }));
 
-            await waitFor(() => {
-                expect(transport.reply).toHaveBeenCalledWith(event, {
-                    room_id: roomId,
-                    delay_id: timeoutDelayId,
+                await waitFor(() => {
+                    expect(transport.reply).toHaveBeenCalledWith(event, {
+                        room_id: roomId,
+                        delay_id: timeoutDelayId,
+                    });
                 });
-            });
 
-            expect(driver.sendDelayedEvent).toHaveBeenCalledWith(
-                event.data.delay,
-                event.data.parent_delay_id,
-                event.data.type,
-                event.data.content,
-                "",
-                roomId,
-            );
-        });
+                expect(driver.sendDelayedEvent).toHaveBeenCalledWith(
+                    event.data.delay ?? null,
+                    event.data.parent_delay_id ?? null,
+                    event.data.type,
+                    event.data.content,
+                    "",
+                    roomId,
+                );
+            },
+        );
 
         it("should reject requests when the driver throws an exception", async () => {
             const roomId = "!room:example.org";
@@ -633,7 +645,7 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: { message: "Error sending event" },
                 });
             });
@@ -673,7 +685,7 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: {
                         message: "Error sending event",
                         matrix_api_error: {
@@ -922,12 +934,12 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: { message: expect.any(String) },
                 });
             });
 
-            expect(driver.cancelScheduledDelayedEvent).not.toBeCalled();
+            expect(driver.cancelScheduledDelayedEvent).not.toHaveBeenCalled();
         });
 
         it("fails to restart delayed events", async () => {
@@ -947,12 +959,12 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: { message: expect.any(String) },
                 });
             });
 
-            expect(driver.restartScheduledDelayedEvent).not.toBeCalled();
+            expect(driver.restartScheduledDelayedEvent).not.toHaveBeenCalled();
         });
 
         it("fails to send delayed events", async () => {
@@ -972,12 +984,12 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: { message: expect.any(String) },
                 });
             });
 
-            expect(driver.sendScheduledDelayedEvent).not.toBeCalled();
+            expect(driver.sendScheduledDelayedEvent).not.toHaveBeenCalled();
         });
 
         it("fails to update delayed events with unsupported action", async () => {
@@ -997,14 +1009,14 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: { message: expect.any(String) },
                 });
             });
 
-            expect(driver.cancelScheduledDelayedEvent).not.toBeCalled();
-            expect(driver.restartScheduledDelayedEvent).not.toBeCalled();
-            expect(driver.sendScheduledDelayedEvent).not.toBeCalled();
+            expect(driver.cancelScheduledDelayedEvent).not.toHaveBeenCalled();
+            expect(driver.restartScheduledDelayedEvent).not.toHaveBeenCalled();
+            expect(driver.sendScheduledDelayedEvent).not.toHaveBeenCalled();
         });
 
         it("can cancel delayed events", async () => {
@@ -1101,7 +1113,7 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: { message: "Error updating delayed event" },
                 });
             });
@@ -1132,7 +1144,7 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: {
                         message: "Error updating delayed event",
                         matrix_api_error: {
@@ -1209,12 +1221,12 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: { message: "Invalid request - missing event type" },
                 });
             });
 
-            expect(driver.sendToDevice).not.toBeCalled();
+            expect(driver.sendToDevice).not.toHaveBeenCalled();
         });
 
         it("fails to send to-device events without event contents", async () => {
@@ -1234,12 +1246,12 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: { message: "Invalid request - missing event contents" },
                 });
             });
 
-            expect(driver.sendToDevice).not.toBeCalled();
+            expect(driver.sendToDevice).not.toHaveBeenCalled();
         });
 
         it("fails to send to-device events without encryption flag", async () => {
@@ -1265,12 +1277,12 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: { message: "Invalid request - missing encryption flag" },
                 });
             });
 
-            expect(driver.sendToDevice).not.toBeCalled();
+            expect(driver.sendToDevice).not.toHaveBeenCalled();
         });
 
         it("fails to send to-device events with any event type", async () => {
@@ -1297,12 +1309,12 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: { message: "Cannot send to-device events of this type" },
                 });
             });
 
-            expect(driver.sendToDevice).not.toBeCalled();
+            expect(driver.sendToDevice).not.toHaveBeenCalled();
         });
 
         it("should reject requests when the driver throws an exception", async () => {
@@ -1333,7 +1345,7 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: { message: "Error sending event" },
                 });
             });
@@ -1371,7 +1383,7 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: {
                         message: "Error sending event",
                         matrix_api_error: {
@@ -1685,12 +1697,12 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: { message: expect.any(String) },
                 });
             });
 
-            expect(driver.readRoomTimeline).not.toBeCalled();
+            expect(driver.readRoomTimeline).not.toHaveBeenCalled();
         });
 
         it("reads state events with a specific state key", async () => {
@@ -1785,12 +1797,12 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: { message: expect.any(String) },
                 });
             });
 
-            expect(driver.readRoomTimeline).not.toBeCalled();
+            expect(driver.readRoomTimeline).not.toHaveBeenCalled();
         });
     });
 
@@ -1806,7 +1818,7 @@ describe("ClientWidgetApi", () => {
 
             emitEvent(new CustomEvent("", { detail: event }));
 
-            expect(transport.reply).toBeCalledWith(event, {
+            expect(transport.reply).toHaveBeenCalledWith(event, {
                 supported_versions: expect.arrayContaining([UnstableApiVersion.MSC3869]),
             });
         });
@@ -1829,12 +1841,12 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     chunk: [createRoomEvent()],
                 });
             });
 
-            expect(driver.readEventRelations).toBeCalledWith(
+            expect(driver.readEventRelations).toHaveBeenCalledWith(
                 "$event",
                 undefined,
                 undefined,
@@ -1872,12 +1884,12 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     chunk: [createRoomEvent(), createRoomEvent({ type: "net.example.test", state_key: "A" })],
                 });
             });
 
-            expect(driver.readEventRelations).toBeCalledWith(
+            expect(driver.readEventRelations).toHaveBeenCalledWith(
                 "$event",
                 undefined,
                 undefined,
@@ -1916,12 +1928,12 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     chunk: [],
                 });
             });
 
-            expect(driver.readEventRelations).toBeCalledWith(
+            expect(driver.readEventRelations).toHaveBeenCalledWith(
                 "$event",
                 "!room-id",
                 "m.reference",
@@ -1944,7 +1956,7 @@ describe("ClientWidgetApi", () => {
 
             emitEvent(new CustomEvent("", { detail: event }));
 
-            expect(transport.reply).toBeCalledWith(event, {
+            expect(transport.reply).toHaveBeenCalledWith(event, {
                 error: { message: "Invalid request - missing event ID" },
             });
         });
@@ -1963,7 +1975,7 @@ describe("ClientWidgetApi", () => {
 
             emitEvent(new CustomEvent("", { detail: event }));
 
-            expect(transport.reply).toBeCalledWith(event, {
+            expect(transport.reply).toHaveBeenCalledWith(event, {
                 error: { message: "Invalid request - limit out of range" },
             });
         });
@@ -1982,7 +1994,7 @@ describe("ClientWidgetApi", () => {
 
             emitEvent(new CustomEvent("", { detail: event }));
 
-            expect(transport.reply).toBeCalledWith(event, {
+            expect(transport.reply).toHaveBeenCalledWith(event, {
                 error: { message: "Unable to access room timeline: !another-room-id" },
             });
         });
@@ -2005,7 +2017,7 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: { message: "Unexpected error while reading relations" },
                 });
             });
@@ -2033,7 +2045,7 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: {
                         message: "Unexpected error while reading relations",
                         matrix_api_error: {
@@ -2064,7 +2076,7 @@ describe("ClientWidgetApi", () => {
 
             emitEvent(new CustomEvent("", { detail: event }));
 
-            expect(transport.reply).toBeCalledWith(event, {
+            expect(transport.reply).toHaveBeenCalledWith(event, {
                 supported_versions: expect.arrayContaining([UnstableApiVersion.MSC3973]),
             });
         });
@@ -2092,7 +2104,7 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     limited: true,
                     results: [
                         {
@@ -2104,7 +2116,7 @@ describe("ClientWidgetApi", () => {
                 });
             });
 
-            expect(driver.searchUserDirectory).toBeCalledWith("foo", undefined);
+            expect(driver.searchUserDirectory).toHaveBeenCalledWith("foo", undefined);
         });
 
         it("should accept all options and pass it to the driver", async () => {
@@ -2138,7 +2150,7 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     limited: false,
                     results: [
                         {
@@ -2155,7 +2167,7 @@ describe("ClientWidgetApi", () => {
                 });
             });
 
-            expect(driver.searchUserDirectory).toBeCalledWith("foo", 5);
+            expect(driver.searchUserDirectory).toHaveBeenCalledWith("foo", 5);
         });
 
         it("should accept empty search_term", async () => {
@@ -2177,13 +2189,13 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     limited: false,
                     results: [],
                 });
             });
 
-            expect(driver.searchUserDirectory).toBeCalledWith("", undefined);
+            expect(driver.searchUserDirectory).toHaveBeenCalledWith("", undefined);
         });
 
         it("should reject requests when the capability was not requested", async () => {
@@ -2197,11 +2209,11 @@ describe("ClientWidgetApi", () => {
 
             emitEvent(new CustomEvent("", { detail: event }));
 
-            expect(transport.reply).toBeCalledWith(event, {
+            expect(transport.reply).toHaveBeenCalledWith(event, {
                 error: { message: "Missing capability" },
             });
 
-            expect(driver.searchUserDirectory).not.toBeCalled();
+            expect(driver.searchUserDirectory).not.toHaveBeenCalled();
         });
 
         it("should reject requests without search_term", async () => {
@@ -2217,11 +2229,11 @@ describe("ClientWidgetApi", () => {
 
             emitEvent(new CustomEvent("", { detail: event }));
 
-            expect(transport.reply).toBeCalledWith(event, {
+            expect(transport.reply).toHaveBeenCalledWith(event, {
                 error: { message: "Invalid request - missing search term" },
             });
 
-            expect(driver.searchUserDirectory).not.toBeCalled();
+            expect(driver.searchUserDirectory).not.toHaveBeenCalled();
         });
 
         it("should reject requests with a negative limit", async () => {
@@ -2240,11 +2252,11 @@ describe("ClientWidgetApi", () => {
 
             emitEvent(new CustomEvent("", { detail: event }));
 
-            expect(transport.reply).toBeCalledWith(event, {
+            expect(transport.reply).toHaveBeenCalledWith(event, {
                 error: { message: "Invalid request - limit out of range" },
             });
 
-            expect(driver.searchUserDirectory).not.toBeCalled();
+            expect(driver.searchUserDirectory).not.toHaveBeenCalled();
         });
 
         it("should reject requests when the driver throws an exception", async () => {
@@ -2263,7 +2275,7 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: { message: "Unexpected error while searching in the user directory" },
                 });
             });
@@ -2292,7 +2304,7 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: {
                         message: "Unexpected error while searching in the user directory",
                         matrix_api_error: {
@@ -2324,7 +2336,7 @@ describe("ClientWidgetApi", () => {
 
             emitEvent(new CustomEvent("", { detail: event }));
 
-            expect(transport.reply).toBeCalledWith(event, {
+            expect(transport.reply).toHaveBeenCalledWith(event, {
                 supported_versions: expect.arrayContaining([UnstableApiVersion.MSC4039]),
             });
         });
@@ -2347,12 +2359,12 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     "m.upload.size": 1000,
                 });
             });
 
-            expect(driver.getMediaConfig).toBeCalled();
+            expect(driver.getMediaConfig).toHaveBeenCalled();
         });
 
         it("should reject requests when the capability was not requested", async () => {
@@ -2366,11 +2378,11 @@ describe("ClientWidgetApi", () => {
 
             emitEvent(new CustomEvent("", { detail: event }));
 
-            expect(transport.reply).toBeCalledWith(event, {
+            expect(transport.reply).toHaveBeenCalledWith(event, {
                 error: { message: "Missing capability" },
             });
 
-            expect(driver.getMediaConfig).not.toBeCalled();
+            expect(driver.getMediaConfig).not.toHaveBeenCalled();
         });
 
         it("should reject requests when the driver throws an exception", async () => {
@@ -2389,7 +2401,7 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: { message: "Unexpected error while getting the media configuration" },
                 });
             });
@@ -2418,7 +2430,7 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: {
                         message: "Unexpected error while getting the media configuration",
                         matrix_api_error: {
@@ -2450,7 +2462,7 @@ describe("ClientWidgetApi", () => {
 
             emitEvent(new CustomEvent("", { detail: event }));
 
-            expect(transport.reply).toBeCalledWith(event, {
+            expect(transport.reply).toHaveBeenCalledWith(event, {
                 supported_versions: expect.arrayContaining([UnstableApiVersion.MSC4039]),
             });
         });
@@ -2477,12 +2489,12 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     content_uri: "mxc://...",
                 });
             });
 
-            expect(driver.uploadFile).toBeCalled();
+            expect(driver.uploadFile).toHaveBeenCalled();
         });
 
         it("should reject requests when the capability was not requested", async () => {
@@ -2498,11 +2510,11 @@ describe("ClientWidgetApi", () => {
 
             emitEvent(new CustomEvent("", { detail: event }));
 
-            expect(transport.reply).toBeCalledWith(event, {
+            expect(transport.reply).toHaveBeenCalledWith(event, {
                 error: { message: "Missing capability" },
             });
 
-            expect(driver.uploadFile).not.toBeCalled();
+            expect(driver.uploadFile).not.toHaveBeenCalled();
         });
 
         it("should reject requests when the driver throws an exception", async () => {
@@ -2523,7 +2535,7 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: { message: "Unexpected error while uploading a file" },
                 });
             });
@@ -2554,7 +2566,7 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: {
                         message: "Unexpected error while uploading a file",
                         matrix_api_error: {
@@ -2616,11 +2628,11 @@ describe("ClientWidgetApi", () => {
 
             emitEvent(new CustomEvent("", { detail: event }));
 
-            expect(transport.reply).toBeCalledWith(event, {
+            expect(transport.reply).toHaveBeenCalledWith(event, {
                 error: { message: "Missing capability" },
             });
 
-            expect(driver.uploadFile).not.toBeCalled();
+            expect(driver.uploadFile).not.toHaveBeenCalled();
         });
 
         it("should reject requests when the driver throws an exception", async () => {
@@ -2641,7 +2653,7 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: { message: "Unexpected error while downloading a file" },
                 });
             });
@@ -2672,7 +2684,7 @@ describe("ClientWidgetApi", () => {
             emitEvent(new CustomEvent("", { detail: event }));
 
             await waitFor(() => {
-                expect(transport.reply).toBeCalledWith(event, {
+                expect(transport.reply).toHaveBeenCalledWith(event, {
                     error: {
                         message: "Unexpected error while downloading a file",
                         matrix_api_error: {
diff --git a/test/WidgetApi-test.ts b/test/WidgetApi-test.ts
index 89273a9..94b6bdc 100644
--- a/test/WidgetApi-test.ts
+++ b/test/WidgetApi-test.ts
@@ -53,7 +53,7 @@ class WidgetTransportHelper {
     /** For ignoring the request sent by {@link WidgetApi.start} */
     private skippedFirstRequest = false;
 
-    public constructor(private channels: TransportChannels) {}
+    public constructor(private readonly channels: TransportChannels) {}
 
     public nextTrackedRequest(): SendRequestArgs | undefined {
         if (!this.skippedFirstRequest) {
@@ -69,7 +69,7 @@ class WidgetTransportHelper {
 }
 
 class ClientTransportHelper {
-    public constructor(private channels: TransportChannels) {}
+    public constructor(private readonly channels: TransportChannels) {}
 
     public trackRequest(action: WidgetApiFromWidgetAction, data: IWidgetApiRequestData): void {
         this.channels.requestQueue.push({ action, data });
@@ -99,7 +99,7 @@ describe("WidgetApi", () => {
 
             const response = clientTrafficHelper.nextQueuedResponse();
             if (response) {
-                window.postMessage(
+                globalThis.postMessage(
                     {
                         ...request,
                         response: response,
@@ -108,14 +108,14 @@ describe("WidgetApi", () => {
                 );
             }
         };
-        window.addEventListener("message", clientListener);
+        globalThis.addEventListener("message", clientListener);
 
         widgetApi = new WidgetApi("WidgetApi-test", "*");
         widgetApi.start();
     });
 
     afterEach(() => {
-        window.removeEventListener("message", clientListener);
+        globalThis.removeEventListener("message", clientListener);
     });
 
     describe("readEventRelations", () => {
@@ -335,10 +335,12 @@ describe("WidgetApi", () => {
                 delay_id: "id",
             } as ISendEventFromWidgetResponseData);
 
-            await expect(widgetApi.sendRoomEvent("m.room.message", {}, "!room-id", 1000, undefined)).resolves.toEqual({
-                room_id: "!room-id",
-                delay_id: "id",
-            });
+            await expect(widgetApi.sendRoomEvent("m.room.message", {}, "!room-id", 1000, "parent-id")).resolves.toEqual(
+                {
+                    room_id: "!room-id",
+                    delay_id: "id",
+                },
+            );
         });
 
         it("sends delayed child action state events", async () => {
@@ -348,7 +350,7 @@ describe("WidgetApi", () => {
             } as ISendEventFromWidgetResponseData);
 
             await expect(
-                widgetApi.sendStateEvent("m.room.topic", "", {}, "!room-id", 1000, undefined),
+                widgetApi.sendStateEvent("m.room.topic", "", {}, "!room-id", 1000, "parent-id"),
             ).resolves.toEqual({
                 room_id: "!room-id",
                 delay_id: "id",
@@ -360,7 +362,7 @@ describe("WidgetApi", () => {
                 error: { message: "An error occurred" },
             } as IWidgetApiErrorResponseData);
 
-            await expect(widgetApi.sendRoomEvent("m.room.message", {}, "!room-id", 1000, undefined)).rejects.toThrow(
+            await expect(widgetApi.sendRoomEvent("m.room.message", {}, "!room-id", 1000)).rejects.toThrow(
                 "An error occurred",
             );
         });
@@ -385,7 +387,7 @@ describe("WidgetApi", () => {
                 },
             } as IWidgetApiErrorResponseData);
 
-            await expect(widgetApi.sendRoomEvent("m.room.message", {}, "!room-id", 1000, undefined)).rejects.toThrow(
+            await expect(widgetApi.sendRoomEvent("m.room.message", {}, "!room-id", 1000)).rejects.toThrow(
                 new WidgetApiResponseError("An error occurred", errorDetails),
             );
         });
@@ -769,3 +771,153 @@ describe("WidgetApi", () => {
         });
     });
 });
+
+describe("capabilities", () => {
+    let widgetApi: WidgetApi;
+
+    beforeEach(() => {
+        widgetApi = new WidgetApi();
+    });
+
+    it("should request single capability", () => {
+        const capability = "org.example.capability";
+        expect(widgetApi.hasCapability(capability)).toBe(false);
+        widgetApi.requestCapability(capability);
+        expect(widgetApi.hasCapability(capability)).toBe(true);
+    });
+
+    it("should request multiple capabilities", () => {
+        const capabilities: string[] = [];
+        for (let i = 1; i <= 3; i++) {
+            capabilities.push(`org.example.capability${i}`);
+        }
+        for (const capability of capabilities) {
+            expect(widgetApi.hasCapability(capability)).toBe(false);
+        }
+        widgetApi.requestCapabilities(capabilities);
+        for (const capability of capabilities) {
+            expect(widgetApi.hasCapability(capability)).toBe(true);
+        }
+    });
+});
+
+describe("postMessage", () => {
+    let widgetApi: WidgetApi;
+    const onHandledRequest = jest.fn();
+
+    let afterMessage: Promise<void>;
+    let afterMessageListener: () => void;
+
+    beforeEach(() => {
+        widgetApi = new WidgetApi();
+        widgetApi.start();
+
+        onHandledRequest.mockClear();
+        widgetApi.transport.on("message", onHandledRequest);
+
+        ({ promise: afterMessage, resolve: afterMessageListener } = Promise.withResolvers<void>());
+        afterMessage = new Promise((resolve) => {
+            afterMessageListener = resolve;
+            globalThis.addEventListener("message", afterMessageListener);
+        });
+    });
+
+    afterEach(() => {
+        globalThis.removeEventListener("message", afterMessageListener);
+        widgetApi.transport.removeAllListeners();
+    });
+
+    async function postMessage(message: unknown): Promise<void> {
+        globalThis.postMessage(message, "*");
+        await afterMessage;
+    }
+
+    it("should handle a valid request", async () => {
+        const action = "a";
+        widgetApi.on(`action:${action}`, (ev: Event) => {
+            ev.preventDefault();
+        });
+
+        await postMessage({
+            api: WidgetApiDirection.ToWidget,
+            requestId: "rid",
+            action,
+            widgetId: "w",
+            data: {},
+        } satisfies IWidgetApiRequest);
+        expect(onHandledRequest).toHaveBeenCalled();
+    });
+
+    it("should drop a request with the wrong direction", async () => {
+        await postMessage({
+            api: WidgetApiDirection.FromWidget,
+            requestId: "rid",
+            action: "a",
+            widgetId: "w",
+            data: {},
+        } satisfies IWidgetApiRequest);
+        expect(onHandledRequest).not.toHaveBeenCalled();
+    });
+
+    it("should drop a request without an action", async () => {
+        await postMessage({
+            api: WidgetApiDirection.ToWidget,
+            requestId: "rid",
+            widgetId: "w",
+            data: {},
+        } satisfies Omit<IWidgetApiRequest, "action">);
+        expect(onHandledRequest).not.toHaveBeenCalled();
+    });
+
+    it("should drop a request without an request ID", async () => {
+        await postMessage({
+            api: WidgetApiDirection.ToWidget,
+            action: "a",
+            widgetId: "w",
+            data: {},
+        } satisfies Omit<IWidgetApiRequest, "requestId">);
+        expect(onHandledRequest).not.toHaveBeenCalled();
+    });
+
+    it("should drop a request without an widget ID", async () => {
+        await postMessage({
+            api: WidgetApiDirection.ToWidget,
+            action: "a",
+            requestId: "rid",
+            data: {},
+        } satisfies Omit<IWidgetApiRequest, "widgetId">);
+        expect(onHandledRequest).not.toHaveBeenCalled();
+    });
+
+    it("should enforce strictOriginCheck", async () => {
+        // the generated MessageEvent will have an empty origin
+        widgetApi.transport.strictOriginCheck = true;
+
+        await postMessage({
+            api: WidgetApiDirection.ToWidget,
+            requestId: "rid",
+            action: "a",
+            widgetId: "w",
+            data: {},
+        } satisfies IWidgetApiRequest);
+        expect(onHandledRequest).not.toHaveBeenCalled();
+    });
+
+    it("should drop a null message", async () => {
+        await postMessage(null);
+        expect(onHandledRequest).not.toHaveBeenCalled();
+    });
+
+    it("should drop a request after having aborted", async () => {
+        widgetApi.transport.stop();
+
+        await postMessage({
+            api: WidgetApiDirection.ToWidget,
+            requestId: "rid",
+            action: "a",
+            widgetId: "w",
+            data: {},
+        } satisfies IWidgetApiRequest);
+        expect(onHandledRequest).not.toHaveBeenCalled();
+    });
+});
diff --git a/tsconfig.json b/tsconfig.json
index e5261eb..075bb62 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -11,7 +11,8 @@
         "declaration": true,
         "types": ["jest"],
         "lib": ["es2020", "dom"],
-        "strict": true
+        "strict": true,
+        "noUnusedLocals": true
     },
     "include": ["./src/**/*.ts"]
 }