fix: simplify MutationSensitiveArray listener system

packages/core/data/dom-events/dom-event.ts

@@ -353,16 +353,17 @@ export class DOMEvent implements Event {
 
 		// Set a listener to clone the array just before any mutations.
 		let listenersLazyCopy: ListenerEntry[] = listenersLive;
-		const doLazyCopy = () => {
+		listenersLive.onMutation = () => (mutation: string, payload?: unknown) => {
+			console.log(`handleEvent "${data.eventName}": doLazyCopy due to "${mutation}"`, payload);
 			// Cloning the array via spread syntax is up to 180 nanoseconds
 			// faster per run than using Array.prototype.slice().
 			listenersLazyCopy = [...listenersLive];
+			listenersLive.onMutation = null;
 		};
-		listenersLive.once(doLazyCopy);
 
-		// Make sure we remove the listener before we exit the function,
-		// otherwise we may wastefully clone the array.
-		const cleanup = () => listenersLive.removeListener(doLazyCopy);
+		// Make sure we clear the callback before we exit the function,
+		// otherwise we may wastefully clone the array on future mutations.
+		const cleanup = () => (listenersLive.onMutation = null);
 
 		for (let i = listenersLazyCopy.length - 1; i >= 0; i--) {
 			const listener = listenersLazyCopy[i];

packages/core/data/mutation-sensitive-array/index.ts

@@ -8,32 +8,11 @@
  * its entire purpose is to be used for performance-sensitive tasks.
  */
 export class MutationSensitiveArray<T> extends Array<T> {
-	private readonly listeners: (() => void)[] = [];
-
-	once(listener: () => void): void {
-		const wrapper = () => {
-			listener();
-			this.removeListener(wrapper);
-		};
-		this.addListener(wrapper);
-	}
-
-	addListener(listener: () => void): void {
-		if (!this.listeners.includes(listener)) {
-			this.listeners.push(listener);
-		}
-	}
-
-	removeListener(listener: () => void): void {
-		const index = this.listeners.indexOf(listener);
-		if (index > -1) {
-			this.listeners.splice(index, 1);
-		}
-	}
+	onMutation: (() => void) | null = null;
 
 	private invalidate(): void {
-		for (const listener of this.listeners) {
-			listener();
+		if (this.onMutation) {
+			this.onMutation();
 		}
 	}