src/vs/base/browser/ui/list/rangeMap.ts/RangeMap/positionAt
class RangeMap:	positionAt(index: number): number {
		if (index < 0) {
			return -1;
		}

		let position = 0;
		let count = 0;

		for (let group of this.groups) {
			const groupCount = group.range.end - group.range.start;
			const newCount = count + groupCount;

			if (index < newCount) {
				return position + ((index - count) * group.size);
			}

			position += groupCount * group.size;
			count = newCount;
		}

		return -1;
	}