src/vs/workbench/contrib/notebook/browser/view/cellParts/foldedCellHint.ts/FoldedCellHint/getHiddenCellHintButton
class FoldedCellHint:	private getHiddenCellHintButton(element: MarkupCellViewModel): HTMLElement {
		const expandIcon = DOM.$('span.cell-expand-part-button');
		expandIcon.classList.add(...CSSIcon.asClassNameArray(Codicon.more));
		this._register(DOM.addDisposableListener(expandIcon, DOM.EventType.CLICK, () => {
			const controller = this._notebookEditor.getContribution<FoldingController>(FoldingController.id);
			const idx = this._notebookEditor.getCellIndex(element);
			if (typeof idx === 'number') {
				controller.setFoldingStateDown(idx, CellFoldingState.Expanded, 1);
			}
		}));

		return expandIcon;
	}