Expected: https://www.screencast.com/t/bvkiEnFk7uhc (as in the Kendo Spreadsheet http://demos.telerik.com/kendo-ui/spreadsheet/index)

Actual: https://www.screencast.com/t/6Vu2f1bR3Oy0 (the tooltip is not visible) https://demos.telerik.com/aspnet-ajax/spreadsheet/examples/overview/defaultcs.aspx

Workaround: 
Add the styles for the tooltip from the Kendo Spreadsheet

html body .RadSpreadsheet .k-tooltip {               

    border-radius: 4px;
    background-image: none, linear-gradient(rgba(255, 255, 255, 0.6) 0px, rgba(255, 255, 255, 0) 100%);
    background-color: rgb(234, 232, 232);
    color: rgb(49, 49, 49);
    box-shadow: rgba(0, 0, 0, 0.3) 0px 4px 6px;
    background-position: 50% 50%;
    border-color: transparent;
    z-index: 12000;
    min-width: 20px;
    text-align: center;
    border-style: solid;
    border-width: 0px;
    padding: 5px 5px 5px 6px;
    background-repeat: repeat-x;
}