While using these themes in one of my projects I discovered that click on the scroll up or down button in popup of combo box closes the ComboBox popup.
I also found the solution for this problem and the solution lies in the ScrollBar's template where increase and decrease repeat buttons are defined.
Focusable="False" IsTabStop="False" are the attributes missing from these buttons which leads popup to close.
So just add these attributes to the IncreaseRepeat and DecreaseRepeat
buttons and problem is solved.
If this is already discovered then just ignore.