src/vs/workbench/contrib/notebook/browser/contrib/fold/folding.ts/FoldingController/onMouseUp
class FoldingController:	onMouseUp(e: INotebookEditorMouseEvent) {
		if (!e.event.target) {
			return;
		}

		const viewModel = this._notebookEditor.viewModel;

		if (!viewModel) {
			return;
		}

		const target = e.event.target as HTMLElement;

		if (target.classList.contains('codicon-notebook-collapsed') || target.classList.contains('codicon-notebook-expanded')) {
			const parent = target.parentElement as HTMLElement;

			if (!parent.classList.contains('notebook-folding-indicator')) {
				return;
			}

			// folding icon

			const cellViewModel = e.target;
			const modelIndex = viewModel.getCellIndex(cellViewModel);
			const state = viewModel.getFoldingState(modelIndex);

			if (state === CellFoldingState.None) {
				return;
			}

			this.setFoldingStateUp(modelIndex, state === CellFoldingState.Collapsed ? CellFoldingState.Expanded : CellFoldingState.Collapsed, 1);
			this._notebookEditor.focusElement(cellViewModel);
		}

		return;
	}