HI~~.
When I show the MDI Form, was added to the Ribbon Tab.
But Owner Form and child form event was used together.
Check the below attach source file.
Hello Min,
Thank you for contacting Infragistics.
As it turns out, this is the expected behavior. Check the link below for more information.
http://help.infragistics.com/doc/WinForms/Current/CLR4.0/?page=WinToolbarsManager_MDI_Toolbar_Merging.html
If you want to avoid this behavior, you can either change the MergeType for the individual tools or provide unique keys for each of the tools.