Hi Martin,
Thank you for using Syncfusion products.
When clicking on the child control inside the PopupControlContainer , you could see that IsShowing method returns true.
Please refer the below sample and let me know if you have any questions.
http://files.syncfusion.com/support/Tools.Windows/7.1.0.30/F80082/Tools_WF_PopupControlContainer_IsShowing.zipIn the above sample, you can see the Output window showing true when clicking on ComboBox inside the PopupControlContainer.
Regards,
Jaya