diff --git a/packages/happy-dom/src/async-task-manager/AsyncTaskManager.ts b/packages/happy-dom/src/async-task-manager/AsyncTaskManager.ts
index e14997256..faa54253a 100644
--- a/packages/happy-dom/src/async-task-manager/AsyncTaskManager.ts
+++ b/packages/happy-dom/src/async-task-manager/AsyncTaskManager.ts
@@ -49,6 +49,10 @@ export default class AsyncTaskManager {
 	 * @param timerID Timer ID.
 	 */
 	public startTimer(timerID: NodeJS.Timeout): void {
+		if (this.waitUntilCompleteTimer) {
+			TIMER.clearTimeout(this.waitUntilCompleteTimer);
+			this.waitUntilCompleteTimer = null;
+		}
 		this.runningTimers.push(timerID);
 	}
 
@@ -58,12 +62,16 @@ export default class AsyncTaskManager {
 	 * @param timerID Timer ID.
 	 */
 	public endTimer(timerID: NodeJS.Timeout): void {
+		if (this.waitUntilCompleteTimer) {
+			TIMER.clearTimeout(this.waitUntilCompleteTimer);
+			this.waitUntilCompleteTimer = null;
+		}
 		const index = this.runningTimers.indexOf(timerID);
 		if (index !== -1) {
 			this.runningTimers.splice(index, 1);
-			if (!this.runningTaskCount && !this.runningTimers.length && !this.runningImmediates.length) {
-				this.resolveWhenComplete();
-			}
+		}
+		if (!this.runningTaskCount && !this.runningTimers.length && !this.runningImmediates.length) {
+			this.resolveWhenComplete();
 		}
 	}
 
@@ -73,6 +81,10 @@ export default class AsyncTaskManager {
 	 * @param immediateID Immediate ID.
 	 */
 	public startImmediate(immediateID: NodeJS.Immediate): void {
+		if (this.waitUntilCompleteTimer) {
+			TIMER.clearTimeout(this.waitUntilCompleteTimer);
+			this.waitUntilCompleteTimer = null;
+		}
 		this.runningImmediates.push(immediateID);
 	}
 
@@ -82,12 +94,16 @@ export default class AsyncTaskManager {
 	 * @param immediateID Immediate ID.
 	 */
 	public endImmediate(immediateID: NodeJS.Immediate): void {
+		if (this.waitUntilCompleteTimer) {
+			TIMER.clearTimeout(this.waitUntilCompleteTimer);
+			this.waitUntilCompleteTimer = null;
+		}
 		const index = this.runningImmediates.indexOf(immediateID);
 		if (index !== -1) {
 			this.runningImmediates.splice(index, 1);
-			if (!this.runningTaskCount && !this.runningTimers.length && !this.runningImmediates.length) {
-				this.resolveWhenComplete();
-			}
+		}
+		if (!this.runningTaskCount && !this.runningTimers.length && !this.runningImmediates.length) {
+			this.resolveWhenComplete();
 		}
 	}
 
@@ -98,6 +114,10 @@ export default class AsyncTaskManager {
 	 * @returns Task ID.
 	 */
 	public startTask(abortHandler?: () => void): number {
+		if (this.waitUntilCompleteTimer) {
+			TIMER.clearTimeout(this.waitUntilCompleteTimer);
+			this.waitUntilCompleteTimer = null;
+		}
 		const taskID = this.newTaskID();
 		this.runningTasks[taskID] = abortHandler ? abortHandler : () => {};
 		this.runningTaskCount++;
@@ -110,27 +130,16 @@ export default class AsyncTaskManager {
 	 * @param taskID Task ID.
 	 */
 	public endTask(taskID: number): void {
+		if (this.waitUntilCompleteTimer) {
+			TIMER.clearTimeout(this.waitUntilCompleteTimer);
+			this.waitUntilCompleteTimer = null;
+		}
 		if (this.runningTasks[taskID]) {
 			delete this.runningTasks[taskID];
 			this.runningTaskCount--;
-			if (this.waitUntilCompleteTimer) {
-				TIMER.clearTimeout(this.waitUntilCompleteTimer);
-			}
-			if (!this.runningTaskCount && !this.runningTimers.length && !this.runningImmediates.length) {
-				// In some cases, microtasks are used by transformed code and waitUntilComplete() is then resolved too early.
-				// To cater for this we use setTimeout() which has the lowest priority and will be executed last.
-				// "10ms" is an arbitrary value, but it seem to be enough when performing many manual tests.
-				this.waitUntilCompleteTimer = TIMER.setTimeout(() => {
-					this.waitUntilCompleteTimer = null;
-					if (
-						!this.runningTaskCount &&
-						!this.runningTimers.length &&
-						!this.runningImmediates.length
-					) {
-						this.resolveWhenComplete();
-					}
-				}, 10);
-			}
+		}
+		if (!this.runningTaskCount && !this.runningTimers.length && !this.runningImmediates.length) {
+			this.resolveWhenComplete();
 		}
 	}
 
@@ -157,11 +166,28 @@ export default class AsyncTaskManager {
 	 * Resolves when complete.
 	 */
 	private resolveWhenComplete(): void {
-		const resolvers = this.waitUntilCompleteResolvers;
-		this.waitUntilCompleteResolvers = [];
-		for (const resolver of resolvers) {
-			resolver();
+		if (this.runningTaskCount || this.runningTimers.length || this.runningImmediates.length) {
+			return;
+		}
+
+		if (this.waitUntilCompleteTimer) {
+			TIMER.clearTimeout(this.waitUntilCompleteTimer);
+			this.waitUntilCompleteTimer = null;
 		}
+
+		// In some cases, microtasks are used by transformed code and waitUntilComplete() is then resolved too early.
+		// To cater for this we use setTimeout() which has the lowest priority and will be executed last.
+		// "10ms" is an arbitrary value, but it seem to be enough when performing many manual tests.
+		this.waitUntilCompleteTimer = TIMER.setTimeout(() => {
+			this.waitUntilCompleteTimer = null;
+			if (!this.runningTaskCount && !this.runningTimers.length && !this.runningImmediates.length) {
+				const resolvers = this.waitUntilCompleteResolvers;
+				this.waitUntilCompleteResolvers = [];
+				for (const resolver of resolvers) {
+					resolver();
+				}
+			}
+		}, 10);
 	}
 
 	/**
diff --git a/packages/happy-dom/src/browser/utilities/BrowserFrameFactory.ts b/packages/happy-dom/src/browser/utilities/BrowserFrameFactory.ts
index 2fb4bd18e..7400b8ee9 100644
--- a/packages/happy-dom/src/browser/utilities/BrowserFrameFactory.ts
+++ b/packages/happy-dom/src/browser/utilities/BrowserFrameFactory.ts
@@ -42,6 +42,12 @@ export default class BrowserFrameFactory {
 			}
 
 			if (!frame.childFrames.length) {
+				if (frame.window && frame.window[PropertySymbol.mutationObservers]) {
+					for (const mutationObserver of frame.window[PropertySymbol.mutationObservers]) {
+						mutationObserver.disconnect();
+					}
+					frame.window[PropertySymbol.mutationObservers] = [];
+				}
 				return frame[PropertySymbol.asyncTaskManager]
 					.destroy()
 					.then(() => {
@@ -60,6 +66,12 @@ export default class BrowserFrameFactory {
 
 			Promise.all(frame.childFrames.slice().map((childFrame) => this.destroyFrame(childFrame)))
 				.then(() => {
+					if (frame.window && frame.window[PropertySymbol.mutationObservers]) {
+						for (const mutationObserver of frame.window[PropertySymbol.mutationObservers]) {
+							mutationObserver.disconnect();
+						}
+						frame.window[PropertySymbol.mutationObservers] = [];
+					}
 					return frame[PropertySymbol.asyncTaskManager].destroy().then(() => {
 						frame[PropertySymbol.exceptionObserver]?.disconnect();
 						if (frame.window) {
diff --git a/packages/happy-dom/src/window/BrowserWindow.ts b/packages/happy-dom/src/window/BrowserWindow.ts
index 8dc923d63..50bdb0032 100644
--- a/packages/happy-dom/src/window/BrowserWindow.ts
+++ b/packages/happy-dom/src/window/BrowserWindow.ts
@@ -494,7 +494,7 @@ export default class BrowserWindow extends EventTarget implements INodeJSGlobal
 	// Used for tracking capture event listeners to improve performance when they are not used.
 	// See EventTarget class.
 	public [PropertySymbol.captureEventListenerCount]: { [eventType: string]: number } = {};
-	public readonly [PropertySymbol.mutationObservers]: MutationObserver[] = [];
+	public [PropertySymbol.mutationObservers]: MutationObserver[] = [];
 	public readonly [PropertySymbol.readyStateManager] = new DocumentReadyStateManager(this);
 	public [PropertySymbol.asyncTaskManager]: AsyncTaskManager | null = null;
 	public [PropertySymbol.location]: Location;
@@ -1319,6 +1319,8 @@ export default class BrowserWindow extends EventTarget implements INodeJSGlobal
 			mutationObserver.disconnect();
 		}
 
+		this[PropertySymbol.mutationObservers] = [];
+
 		// Disconnects nodes from the document, so that they can be garbage collected.
 		for (const node of this.document[PropertySymbol.childNodes].slice()) {
 			// Makes sure that something won't be triggered by the disconnect.
diff --git a/packages/happy-dom/test/nodes/document/Document.test.ts b/packages/happy-dom/test/nodes/document/Document.test.ts
index 97c67d4cc..0a726b051 100644
--- a/packages/happy-dom/test/nodes/document/Document.test.ts
+++ b/packages/happy-dom/test/nodes/document/Document.test.ts
@@ -1255,7 +1255,7 @@ describe('Document', () => {
 					expect((<Event>readyChangeEvent).target).toBe(document);
 					expect(document.readyState).toBe(DocumentReadyStateEnum.complete);
 					resolve(null);
-				}, 1);
+				}, 20);
 			});
 		});