Hi Thomas,
Anytime a control "blurs" focus (moves it back to the main focus scope), then that will trigger the PopupButton's popup to close. WPF Buttons in general by default do blur focus when clicked unless they are coded with some StaysOpenOnClick-like functionality. It's hard to say what is causing the popup to close in your scenario without a sample project to debug. If you'd like us to look into it further, please make a new simple sample project that shows it happening and send that to our support address. Be sure to mention this thread in your email, remove the bin/obj fodlers from the ZIP, and rename the .zip file extension so it doesn't get spam blocked. Thanks!