yeah, can verify that there's a different border used. But no, it's not the tree editor that's used - editing is handled completely by the table-part of the tree table (and the DefaultCellEditor in its role a TableCellEditor).
Problem is that TreeTableCellEditor extends DefaultCellEditor while the other default editors in the J/X/Table extend GenericEditor (which in turn DefaultCellEditor). GenericEditor behaves rather rude in installing a black line border on everything.
Hmm ... will try to extend GenericEditor - and cross fingers <g>