Best way to keep popup button open when it has a checkbox

I have a popup menu button and I am wanting some of its menu items to be check boxes. However, I can't seem to find a good way to keep the popup button's menu open when I click a check box. What is the best way to accomplish this? My code is something like this (btw, I'm not hard set on having a "checkbox menu item"... as long as there is a checkbox in my popup button's menu that, when checked/unchecked, doesn't cause the popup menu to close):

<ribbon:PopupButton Label="Search" StaysOpenOnClick="True">
<ribbon:Menu>
<ribbon:ComboBox MinWidth="100" Label="Search:" IsEditable="True">
<ComboBoxItem>Previous search item 1</ComboBoxItem>
<ComboBoxItem>Previous search item 2</ComboBoxItem>
<ComboBoxItem>Previous search item 3</ComboBoxItem>
</ribbon:ComboBox>
<ribbon:Separator Label="Search Options" />
<ribbon:CheckBox Label="Option 1" IsChecked="True" />
<ribbon:CheckBox Label="Option 2" IsChecked="False" />
</ribbon:Menu>
</ribbon:PopupButton>