extensions/markdown-language-features/src/util/topmostLineMonitor.ts/TopmostLineMonitor/constructor
class TopmostLineMonitor:	constructor() {
		super();

		if (vscode.window.activeTextEditor) {
			const line = getVisibleLine(vscode.window.activeTextEditor);
			this.setPreviousTextEditorLine({ uri: vscode.window.activeTextEditor.document.uri, line: line ?? 0 });
		}

		this._register(vscode.window.onDidChangeTextEditorVisibleRanges(event => {
			if (isMarkdownFile(event.textEditor.document)) {
				const line = getVisibleLine(event.textEditor);
				if (typeof line === 'number') {
					this.updateLine(event.textEditor.document.uri, line);
					this.setPreviousTextEditorLine({ uri: event.textEditor.document.uri, line: line });
				}
			}
		}));
	}