extensions/markdown-language-features/src/commands/showSource.ts/ShowSourceCommand/execute
class ShowSourceCommand:	public execute() {
		const { activePreviewResource, activePreviewResourceColumn } = this._previewManager;
		if (activePreviewResource && activePreviewResourceColumn) {
			return vscode.workspace.openTextDocument(activePreviewResource).then(document => {
				return vscode.window.showTextDocument(document, activePreviewResourceColumn);
			});
		}
		return undefined;
	}