src/vs/editor/common/model/prefixSumComputer.ts/ConstantTimePrefixSumComputer/getIndexOf
class ConstantTimePrefixSumComputer:	public getIndexOf(sum: number): PrefixSumIndexOfResult {
		this._ensureValid();
		const idx = this._indexBySum[sum];
		const viewLinesAbove = idx > 0 ? this._prefixSum[idx - 1] : 0;
		return new PrefixSumIndexOfResult(idx, sum - viewLinesAbove);
	}