The default font size on the client is not saved explicitly on the Excel file, so the Excel defaults take over (11px Calibri), so upon subsequent import all cells will have that font.

Steps to repro:

    go to https://demos.telerik.com/aspnet-ajax/spreadsheet/examples/import-export/defaultcs.aspx 
    export the file
    import the file

Expected: nothing changes

Actual: all cells are 11px Calibri
Workaround: Set a default font when the cell font is null
 
public override void SaveWorkbook(Workbook workbook)
{
    string path = "C:\\path-to-output.xlsx";
     
    for (int i = 0; i < workbook.Sheets.Count; i++)
    {
        for (int j = 0; j < workbook.Sheets[i].Rows.Count; j++)
        {
            for (int l = 0; l < workbook.Sheets[i].Rows[i].Cells.Count; l++)
            {
                string fontFamily = workbook.Sheets[i].Rows[i].Cells[l].FontFamily;
                workbook.Sheets[i].Rows[i].Cells[l].FontFamily = !string.IsNullOrEmpty(fontFamily) ? fontFamily : "Courier New";
 
                double? fontSize = workbook.Sheets[i].Rows[i].Cells[l].FontSize;
                workbook.Sheets[i].Rows[i].Cells[l].FontSize = fontSize.HasValue ? fontSize : 12;
            }
        }
    }
    workbook.Save(path);
}
 

The lines shifts badly after line 10 

You think you are on line 17  but the cursor is on line 16.  Very annoying bug for those using this control.  Please fix and give us an update.

Look the attached file.

A workaround is using directly the Kendo Spreadsheet widget as this will allow you to use its latest version that does not manifest this issue: https://demos.telerik.com/kendo-ui/spreadsheet/server-side-import-export

Repro steps

    Go to https://demos.telerik.com/aspnet-ajax/spreadsheet/examples/import-export/defaultcs.aspx
    Go to an empty cell
    type some text
    change its font size (e.g., to 18)
    save the file
    import the file you just saved

Expected: nothing changes

Actual: (some) cells change their height and font size, not just the cell that was edited above

If you type 15 into the font size combo box and click away, a Javascript error is thrown and the font size of the selected cell does not change. If instead you type 16 and click away, the cell changes correctly. 16 is an option in the drop-down but 15 is not.

Error message:

RadSpreadsheetScripts.js:196 Uncaught TypeError: Cannot read property 'get_text' of null


The font size combo box works correctly in the Kendo spreadsheet demo (both with the current and the 2017.1.124 Kendo version ). You can also press enter to commit your typed font size in the Kendo widget, but not in RadSpreadsheet.

Steps to reproduce:
1. Open https://demos.telerik.com/aspnet-ajax/spreadsheet/examples/overview/defaultcs.aspx
2. Click inside the font -size dropdown
3. Type 15 and click outside the SpreadSheet

Result: A JavaScript error is thrown.

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;
}

related to https://github.com/telerik/kendo-ui-core/issues/3519
workaround for rounding function is using Custom Functions http://docs.telerik.com/kendo-ui/controls/data-management/spreadsheet/custom-functions

kendo.spreadsheet.defineFunction("ROUND", function (x) {
    return Math.round(x);
}).args([
    ["x", "number"]
]);

Cell cell = new Cell() { Index = 0, Formula = "=ROUND(1.2)", Bold = true, Background = "#fef0cd", Format = "$0" };

http://screencast.com/t/FJjlN3yw

The error message reads
'ContextMenus' could not be initialized. Details: Object reference on set to an instance of an object.

Allow to set the filename for the radspeadsheet control when a client API SaveAs is performed e.g. MyRadSpreadsheetControl.saveAsExcel("myfilename.xlsx");