The MRML node combobox can have extra items on the top or bottom of the node list. There are built-in extra items, such as None, Add, etc. When these are enabled, then a separator appears between the nodes and the items. However, when no built-in extra item is enabled, just custom extra items are added, then the separator did not show up.
requested to merge github/fork/cpinter/mrmlnodecombobox-separator-for-custom-items into master