Page Down does not work with selectable VScrollTable when there's a horizontal scrollbar

Originally by T-Gergely

This code fails when there's a horizontal scrollbar:

                    VScrollTableRow lastVisibleRowInViewPort = scrollBody
                            .getRowByRowIndex(firstRowInViewPort
                                    + getFullyVisibleRowCount() - 1);
                    if (lastVisibleRowInViewPort != null
                            && lastVisibleRowInViewPort != focusedRow) {

Client height should be divided instead of offset height:

    private int getFullyVisibleRowCount() {
        return (int) (scrollBodyPanel.getOffsetHeight() / scrollBody
                .getRowHeight());
    }

I don't know GWT, but the below seems to work:

    private int getFullyVisibleRowCount() {
        return (int) (DOM.getElementPropertyInt(scrollBodyPanel.getElement(),
          "clientHeight") / scrollBody.getRowHeight());
    }

Imported from https://dev.vaadin.com/ issue #13338

Start by locating getFullyVisibleRowCount and the selectable VScrollTable Page Down handling described in the issue. Reproduce the case with a horizontal scrollbar, inspect the existing row-count calculation, and verify that Page Down selects the expected visible row without the scrollbar affecting the calculation.