src/vs/editor/common/viewLayout/linesLayout.ts/LinesLayout/getWhitespacesAccumulatedHeight
class LinesLayout:	public getWhitespacesAccumulatedHeight(index: number): number {
		this._checkPendingChanges();
		index = index | 0;

		let startIndex = Math.max(0, this._prefixSumValidIndex + 1);
		if (startIndex === 0) {
			this._arr[0].prefixSum = this._arr[0].height;
			startIndex++;
		}

		for (let i = startIndex; i <= index; i++) {
			this._arr[i].prefixSum = this._arr[i - 1].prefixSum + this._arr[i].height;
		}
		this._prefixSumValidIndex = Math.max(this._prefixSumValidIndex, index);
		return this._arr[index].prefixSum;
	}