When you try implementing a cell who's height is dependent on its width for example wrapped text. It failed to layout correctly because VirtualFlow always passes in "-1" for the width to prefHeight() method. This should be simply reproducible by creating a ListView where each item is a long String with a paragraph of text. Then setWrap(true) on all the cells.

Looking at VirtualFlow line 1548 there are these two method.

    private double getCellPrefLength(IndexedCell cell) {
        return isVertical() ?
            cell.prefHeight(-1)
            : cell.prefWidth(-1);
    }

    /**
     * Gets the breadth of a specific cell
     */
    private double getCellBreadth(Cell cell) {
        return isVertical() ?
            cell.prefWidth(-1)
            : cell.prefHeight(-1);
    }

I think they need to pass in the width or height. They will need to check getContentBias() of the cell first to see if it needs it as passing "-1" is faster as its cached and we do not want to loose performance when ContentBias is not needed.