chore: import upstream snapshot with attribution

extensions/markdown-language-features/preview-src/loading.ts

@@ -0,0 +1,42 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+import { MessagePoster } from './messaging';
+
+export class StyleLoadingMonitor {
+	private _unloadedStyles: string[] = [];
+	private _finishedLoading: boolean = false;
+
+	private _poster?: MessagePoster;
+
+	constructor() {
+		const onStyleLoadError = (event: any) => {
+			const source = event.target.dataset.source;
+			this._unloadedStyles.push(source);
+		};
+
+		window.addEventListener('DOMContentLoaded', () => {
+			for (const link of document.getElementsByClassName('code-user-style') as HTMLCollectionOf<HTMLElement>) {
+				if (link.dataset.source) {
+					link.onerror = onStyleLoadError;
+				}
+			}
+		});
+
+		window.addEventListener('load', () => {
+			if (!this._unloadedStyles.length) {
+				return;
+			}
+			this._finishedLoading = true;
+			this._poster?.postMessage('previewStyleLoadError', { unloadedStyles: this._unloadedStyles });
+		});
+	}
+
+	public setPoster(poster: MessagePoster): void {
+		this._poster = poster;
+		if (this._finishedLoading) {
+			poster.postMessage('previewStyleLoadError', { unloadedStyles: this._unloadedStyles });
+		}
+	}
+}