Hello Ying,

I have been investigating this issue and have put together a sample project using the relevant code that you have provided in an attempt to reproduce this behavior, but I cannot seem to reproduce it.

I have attached the sample project I used to test this.  Please test this project on your PC; whether or not it works correctly may help indicate the nature of this problem.

If the project does not work correctly, this indicates either a problem possibly specific to your environment, or a difference in the DLL versions we are using.  My test was performed using version 17.2.20172.1000 in Infragistics for WPF 2017.2.

If the project does show the product feature working correctly, this indicates a possible problem in the code of your application.  It will help if you can provide a small, isolated sample application that demonstrates the behavior you are seeing.

Or, if this sample project is not an accurate demonstration of what you're trying to do, please feel free to modify it and send it back, or send a small sample project of your own if you have one.

Please let me know if you have any other questions or concerns on this matter.