Add Event.defer with hidden experimental setting

Part of #161622

src/vs/base/common/event.ts

@@ -53,6 +53,44 @@ export namespace Event {
 		}
 	}
 
+	// TODO: This boolean will go away once it's stable
+	/**
+	 * Whether to enable the {@link Event.defer}.
+	 */
+	// eslint-disable-next-line prefer-const
+	export let enableDefer: boolean = false;
+
+	/**
+	 * Given an event, returns another event which defers the listener to a later task via a shared `setTimeout`. This
+	 * is useful for deferring non-critical work (eg. general UI updates) to ensure it does not block critical work (eg.
+	 * latency of keypress to text rendered).
+	 *
+	 * *NOTE* when using this it's important to consider race conditions that could arise when using related deferred
+	 * and non-deferred events.
+	 */
+	export function defer<T>(event: Event<T>): Event<T> {
+		const listeners: { listener: (e: T) => any; thisArgs?: any; disposables?: IDisposable[] | DisposableStore }[] = [];
+		let primaryListener: IDisposable | undefined;
+		return (listener, thisArgs = null, disposables?) => {
+			listeners.push({ listener, thisArgs, disposables });
+			if (!primaryListener) {
+				primaryListener = event(e => {
+					if (enableDefer) {
+						setTimeout(() => {
+							for (const l of listeners) {
+								l.listener.call(l.thisArgs, e);
+							}
+						}, 0);
+					} else {
+						for (const l of listeners) {
+							l.listener.call(l.thisArgs, e);
+						}
+					}
+				});
+			}
+			return primaryListener;
+		};
+	}
 
 	/**
 	 * Given an event, returns another event which only fires once.

src/vs/workbench/contrib/performance/browser/inputLatency.ts

@@ -0,0 +1,27 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+import { Event } from 'vs/base/common/event';
+import { Disposable } from 'vs/base/common/lifecycle';
+import { IConfigurationService } from 'vs/platform/configuration/common/configuration';
+import { IWorkbenchContribution } from 'vs/workbench/common/contributions';
+
+export class InputLatencyContrib extends Disposable implements IWorkbenchContribution {
+	constructor(
+		@IConfigurationService private readonly _configurationService: IConfigurationService
+	) {
+		super();
+		this._register(_configurationService.onDidChangeConfiguration(() => this._refresh()));
+		this._refresh();
+	}
+
+	private _refresh() {
+		const value = this._configurationService.getValue<boolean>('performance.inputLatency') || false;
+		if (Event.enableDefer !== value) {
+			console.warn('Event.enableDefer = ', value);
+			Event.enableDefer = value;
+		}
+	}
+}

src/vs/workbench/contrib/performance/browser/performance.contribution.ts

@@ -15,6 +15,7 @@ import { PerfviewContrib, PerfviewInput } from 'vs/workbench/contrib/performance
 import { IEditorService } from 'vs/workbench/services/editor/common/editorService';
 import { InstantiationService, Trace } from 'vs/platform/instantiation/common/instantiationService';
 import { EventProfiling } from 'vs/base/common/event';
+import { InputLatencyContrib } from 'vs/workbench/contrib/performance/browser/inputLatency';
 
 // -- startup performance view
 
@@ -39,6 +40,14 @@ Registry.as<IEditorFactoryRegistry>(EditorExtensions.EditorFactory).registerEdit
 );
 
 
+// -- input latency reduction experiment
+
+Registry.as<IWorkbenchContributionsRegistry>(Extensions.Workbench).registerWorkbenchContribution(
+	InputLatencyContrib,
+	LifecyclePhase.Ready
+);
+
+
 registerAction2(class extends Action2 {
 
 	constructor() {