1. Having a combo box in NatTable that supports multiple selection
2. A filter that can handle this
Currently NatTable doesn't support such a combo box. I worked on that a few months ago, but haven't got time to merge this into NatTable core yet. I will do this in the next few weeks.
After that we can have a look on how filters are able to react on that.
So I am only able to tell you that you have to wait some time to get that feature. At least this is what I can tell you.
as I'm planning to adjust and commit it to the NatTable, I can send you what I have. Posting here would be too much, as I reimplemented the whole NatCombo and ComboBoxCellEditor. The main deal is to merge what currently exists with the one I implemented to have one control that can be configured for single/multi selection.
with the major edit refactoring I committed a ComboBoxCellEditor that supports multi selection. Feel free to check if it is working for you. It should also cover the cases on rendering like you wrote me by email.
Yes it is. Look at the ComboBoxCellEditor. There should be a constructor parameter for enabling multiselect. But I plan to push a change to that so it is not set by constructor but by setter instead soon.