getDesiredScrollPositionFromOffsetPaged(offset) {
        if (!this._computedIsNeeded) {
            // no need for a slider
            return 0;
        }
        const correctedOffset = offset - this._arrowSize; // compensate if has arrows
        let desiredScrollPosition = this._scrollPosition;
        if (correctedOffset < this._computedSliderPosition) {
            desiredScrollPosition -= this._visibleSize; // page up/left
        }
        else {
            desiredScrollPosition += this._visibleSize; // page down/right
        }
        return desiredScrollPosition;
    }
Compute a desired `scrollPosition` from if offset is before or after the slider position.
If offset is before slider, treat as a page up (or left).  If after, page down (or right).
`offset` and `_computedSliderPosition` are based on the same coordinate system.
`_visibleSize` corresponds to a "page" of lines in the returned coordinate system.