Hi,
Please try commenting out the code in the mousedown event. Since you have associated the popupmenu to GDBG and allowed it to show only on the last column using before_popup, there is no need to make the popupmenu to show when mouse down occurs in the GDBG.
I have attached a sample that demonstrates this completely. Please refer to it and let me know if this meets your requirements.
Thanks for using Syncfusion products.
Regards,
Mano
GDBG_Popupmenu.zip