extensions/markdown-language-features/src/preview/topmostLineMonitor.ts/TopmostLineMonitor/updateLine
class TopmostLineMonitor:	public updateLine(
		resource: vscode.Uri,
		line: number
	) {
		if (!this._pendingUpdates.has(resource)) {
			// schedule update
			setTimeout(() => {
				if (this._pendingUpdates.has(resource)) {
					this._onChanged.fire({
						resource,
						line: this._pendingUpdates.get(resource) as number
					});
					this._pendingUpdates.delete(resource);
				}
			}, this._throttle);
		}

		this._pendingUpdates.set(resource, line);
	}