ToolStripDropDown Menu remains open even on click of other windows

Hi,
I am using a toolstripDropdownbutton on the toolstrip to populate the
menuItems.
The toolstrip is on the Customtaskpane of the window.
Problem comes when the focus is on the customtaskpane, user opens a dropdown
menu and without selecting any menu from the dropdown click on the other part
of the window. The dropdown remains open and even the user switch to other
window it remains open and does not hide. It only hide when user click on the
dropdownbutton itself.
Any help will be appreciated...