In Cell In Cell editing means the contents of the cell will be cleared and the editor will appear inside the cell.
If you are using full row edit, then you are blocked from using popup editors.
This does not mean that you cannot show a popup from your 'in cell' editor - you are free to do that - however the responsibility of showing and hiding the popup belongs with your editor.
The popup will behave like a menu in that any mouse interaction outside of the popup will close the popup.
The popup will appear over the cell, however it will not change the contents of the cell.
Cell Renderer's and Cell Editors, the former for showing the data, the latter for editing the data.
If your application is for showing data only, such as a reporting application, then you will not need to use cell editors.
The default editor is used if you do not provide a cell editor.
If you have An editor can be in a popup or in cell.
In other scenarios, editing groups does make sense.