OutlineView have it's own private implementation of OutlineColumn therefore it's not possible to override isSortingAllowed(). The implementation takes the SortableColumn property from Node.Property of the column, but there's no Node.Property for the tree column. Therefore there's no way to change the sortability of the tree column.
Created attachment 92550 [details]
The API change.
Please review this simple API change.
I've rewritten OutlineViewOutlineColumn.isSortingAllowed() and added OutlineView.setTreeSortable() to allow to change the sortability of the tree column.
Thanks for the API review.
Fixed in changeset: 156091:95cceacc743a
Integrated into 'main-golden', will be available in build *200912220201* on http://bits.netbeans.org/dev/nightly/ (upload may still be in progress)
Log: #178613 - Allow to set the tree column as not sortable.