Currently, changing the sheets performs heavy calculations on the UI thread. This does not allow to perform any other UI task and the busy indicator cannot be shown before all layers are rendered.

The background should be stretched along with the item itself. It is currently with specified width.
Workaround: Modify the style and change the width of the tab item. A sample is attached.

An ArgumentNullException is thrown when you try to open a document containing relationship namespace declared in the tag.

<sheet name="Management Position View" sheetId="1" d3p1:id="rId1" xmlns:d3p1="http://schemas.openxmlformats.org/officeDocument/2006/relationships" />

The exception is thrown in the QuarterAutoFillGenerator TryParse method due to the unexpected length of the value.

Workaround: Do not use SetValueAsText method by passing an empty string to it. Use SetValue or ClearValue instead.

At the moment the theme of the RadSpreadsheetRibbon can be set only by setting the theme of the entire application. The individual setting (using telerik:StyleManager.Theme="Windows8") should also be supported.

When the size of the columns is changed using the ColumnSelection.SetWidth(*) method does not update the layout, so subsequent uses of the layout are not calculated correctly. For example BringIntoView method works incorrectly. 

Workaround: call RadSpreadsheet.UpdateLayout() after setting the width.

Cell borders below the cells filled with data are not exported to XLSX.

When a sheet that has a new line in a text value is exported and the resulting file is opened in Excel, the value is displayed without a new line. If you enter edit mode and then exit (without confirming the value by pressing enter) the new line will appear. The reason for this behavior is that when there is a new line in a text value Excel automatically applies wrap text on the cell while RadSpreadsheet does not do that. A possible workaround is to apply wrap text on new values with new line in them.

Implement mechanism for disabling, enabling or changing of the commands of the CellEditor classes, like Copy and Paste.

When filtering a big range of values, the performance is low. The whole UI is re-created.

The exception is thrown when the color for the borders has been previously set using the colors from the predefined palette. The next attempt to set a border color using the ShowMoreColorsDialog leads to error. A message appears to the user saying "Specified cast is not valid".

When trying to set the VisibleSize property of RadSpreadsheet on the ActiveSheetChanged event a null exception is thrown in the UpdateSheet method of RadSheetEditorBase. 

Possible workaround is to record the visible size at the SheetChanged event and set it at a later time, e.g. using a timer.

When a row is removed through code and an image is created immediately after that in the same method, the UI does not update. The change is made in the model, but not visualized. If you scroll up and down the change is reflected in the UI.

The SUMIFS function adds all of its arguments that meet multiple criteria.

A list of the supported functions is available at http://docs.telerik.com/devtools/document-processing/libraries/radspreadprocessing/features/formulas/functions

This function can be implemented as a custom function. Check the following resources for more details on how to achieve that:

- http://docs.telerik.com/devtools/document-processing/libraries/radspreadprocessing/features/formulas/custom-functions
- https://github.com/telerik/xaml-sdk/tree/master/Spreadsheet/WPF/CustomFunctions

Selecting a column with two or more merged cells, selects not only the selected column but the rest of columns that the merged cells are from. The behavior when working with rows is analogical.

The COUNTIF function is already available in RadSpreadsheet.

- Async parse of single xml file to speed-up the import
- Allow to import only part of the file
- Respect the widths of the columns and row heights and do not measure them first time in the WorksheetLayout or measure them for the current Viewport only
- Inserting rows/columns also need improvements in terms of performance.

if you use Excel, you use a lot of formulas. 
But In Telerik WPF there isn't enough formulas.(even like SUMIFS , SUBTOTAL) 

But I search for other library like Kendo-UI, there is more fomulas than WPF. 
Why are these different? 

Please make this formulas in your library.

The class would be used for customization of the printing behavior of the control.