Revert "Fixes #224943" (#225161)

This reverts commit adbf397.

src/vs/editor/common/services/languageService.ts

@@ -11,7 +11,6 @@ import { ILanguageNameIdPair, ILanguageSelection, ILanguageService, ILanguageIco
 import { firstOrDefault } from 'vs/base/common/arrays';
 import { ILanguageIdCodec, TokenizationRegistry } from 'vs/editor/common/languages';
 import { PLAINTEXT_LANGUAGE_ID } from 'vs/editor/common/languages/modesRegistry';
-import { IObservable, observableFromEvent } from 'vs/base/common/observable';
 
 export class LanguageService extends Disposable implements ILanguageService {
 	public _serviceBrand: undefined;
@@ -151,13 +150,51 @@ export class LanguageService extends Disposable implements ILanguageService {
 }
 
 class LanguageSelection implements ILanguageSelection {
-	private readonly _value: IObservable<string>;
-	public readonly onDidChange: Event<string>;
 
-	constructor(onDidChangeLanguages: Event<void>, selector: () => string) {
-		this._value = observableFromEvent(this, onDidChangeLanguages, () => selector());
-		this.onDidChange = Event.fromObservable(this._value);
+	public languageId: string;
+
+	private _listener: IDisposable | null = null;
+	private _emitter: Emitter<string> | null = null;
+
+	constructor(
+		private readonly _onDidChangeLanguages: Event<void>,
+		private readonly _selector: () => string
+	) {
+		this.languageId = this._selector();
+	}
+
+	private _dispose(): void {
+		if (this._listener) {
+			this._listener.dispose();
+			this._listener = null;
+		}
+		if (this._emitter) {
+			this._emitter.dispose();
+			this._emitter = null;
+		}
 	}
 
-	public get languageId(): string { return this._value.get(); }
+	public get onDidChange(): Event<string> {
+		if (!this._listener) {
+			this._listener = this._onDidChangeLanguages(() => this._evaluate());
+		}
+		if (!this._emitter) {
+			this._emitter = new Emitter<string>({
+				onDidRemoveLastListener: () => {
+					this._dispose();
+				}
+			});
+		}
+		return this._emitter.event;
+	}
+
+	private _evaluate(): void {
+		const languageId = this._selector();
+		if (languageId === this.languageId) {
+			// no change
+			return;
+		}
+		this.languageId = languageId;
+		this._emitter?.fire(this.languageId);
+	}
 }