src/vs/editor/contrib/folding/browser/foldingRanges.ts/FoldingRegions/getParentIndex
class FoldingRegions:	public getParentIndex(index: number) {
		this.ensureParentIndices();
		let parent = ((this._startIndexes[index] & MASK_INDENT) >>> 24) + ((this._endIndexes[index] & MASK_INDENT) >>> 16);
		if (parent === MAX_FOLDING_REGIONS) {
			return -1;
		}
		return parent;
	}