uicontrol('style','popup') do not support separator lines. You can add an entry which is a bunch of dashes but that will affect the count of the entries.
There is a trick you might be interested in: each individual cell in the cell array of strings will be parsed as HTML if it begins with '<HTML>'. For example,
uicontrol('string', {'hello', 'ab cd', '<HTML>ab cd', '<HTML>ab<br>cdef', '<HTML>ab<br>--------', 'pqrs'})
the first nbsp will be treated as pure text because it is not within an HTML marker; the one on the next line will be. The '<br>' works on the line after that leading to the '<br>' followed by a line of dashes to allow a visual separator. The drawback is that when the item is chosen, the dashes will show up as part of the current selection.
