positionAt(index) {
        if (index < 0) {
            return -1;
        }
        let position = 0;
        let count = 0;
        for (const group of this.groups) {
            const groupCount = group.range.end - group.range.start;
            const newCount = count + groupCount;
            if (index < newCount) {
                return this._paddingTop + position + ((index - count) * group.size);
            }
            position += groupCount * group.size;
            count = newCount;
        }
        return -1;
    }
Returns the start position of the item at the given index.