function findPageByRectangles($element, spatialVerticalOffset) {
        var visibleContentOffsets = getVisibleContentOffsets();
        var elementRectangles = getNormalizedRectangles($element, visibleContentOffsets);
        var clientRectangles  = elementRectangles.clientRectangles;
        if (clientRectangles.length === 0) { // elements with display:none, etc.
            return null;
        }

        var isRtl = isPageProgressionRightToLeft();
        var isVwm = isVerticalWritingMode();
        var columnFullWidth = getColumnFullWidth();

        var frameHeight = $iframe.height();
        var frameWidth  = $iframe.width();

        if (spatialVerticalOffset) {
            trimRectanglesByVertOffset(clientRectangles, spatialVerticalOffset,
                frameHeight, columnFullWidth, isRtl, isVwm);
        }

        var firstRectangle = _.first(clientRectangles);
        if (clientRectangles.length === 1) {
            adjustRectangle(firstRectangle, {
                height: frameHeight, width: frameWidth
            }, columnFullWidth, isRtl, isVwm);
        }

        var pageIndex;

        if (isVwm) {
            var topOffset = firstRectangle.top;
            pageIndex = Math.floor(topOffset / frameHeight);
        } else {
            var leftOffset = firstRectangle.left;
            if (isRtl) {
                leftOffset = (columnFullWidth * (options.paginationInfo ? options.paginationInfo.visibleColumnCount : 1)) - leftOffset;
            }
            pageIndex = Math.floor(leftOffset / columnFullWidth);
        }

        if (pageIndex < 0) {
            pageIndex = 0;
        }
        else if (pageIndex >= (options.paginationInfo ? options.paginationInfo.columnCount : 1)) {
            pageIndex = (options.paginationInfo ? (options.paginationInfo.columnCount - 1) : 0);
        }

        return pageIndex;
    }
Finds a page index (0-based) for a specific element.
Calculations are based on rectangles retrieved with getClientRects() method.

@param {jQuery} $element
@param {number} spatialVerticalOffset
@returns {number|null}