Hi Nick,
Thank you for using Syncfusion products.
Query #1:The added menu items could display correctly, but when I right click the work area of the document ( not the document header ), only the new two menu items displayed in the context menu, it's strange.
The behavior of our DockingManager control is displaying both the default and custom context menu items on right clicking document tab header. On right clicking the other part of document tab, only the custom context menu items are displayed. That’s why custom context menu items are displayed when you right click the work area of the document.
Query #2: How to let the customized menu item invisible when I click other place not the document header. Is there any easy way to do that? Or it's a defect of DockingManager?
To prevent the custom context menu items from opening on right click at work area of the document, we suggest you to handle the PreviewMouseRightButtonUp event of each document child element in DockingManager. You can download the sample for the same from the following link:
Sample:
StrangeContextMenuItem_ModifiedRegards,
Vijayalakshmi V.R.