getDesiredScrollPositionFromOffset(offset) {
        if (!this._computedIsNeeded) {
            // no need for a slider
            return 0;
        }
        const desiredSliderPosition = offset - this._arrowSize - this._computedSliderSize / 2;
        return Math.round(desiredSliderPosition / this._computedSliderRatio);
    }
Compute a desired `scrollPosition` such that `offset` ends up in the center of the slider.
`offset` is based on the same coordinate system as the `sliderPosition`.