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

		this.pendingUpdates.set(key, line);
	}