NSTableCellView does not colour its background correctly

In an NSTableView (Appkit), I need to colour a cell background when it is selected.

That works OK, except that the colour does not span the full cell width, nor even the text itself:

The tableView structure is very basic:

I see there is a TextCell at the end that cannot be deleted. What is this ?

And the colouring as well:

    func tableView(_ tableView: NSTableView, viewFor tableColumn: NSTableColumn?, row: Int) -> NSView? {
  
                let p = someDataSource[row]

                if let cellView = tableView.makeView(withIdentifier: NSUserInterfaceItemIdentifier(rawValue: "Cell"), owner: self) {
                    (cellView as! NSTableCellView).textField?.stringValue = p
                    
                    if selected[row] { 
                        (cellView as! NSTableCellView).backgroundColor = theLightBlueColor
                    } else {
                        (cellView as! NSTableCellView).backgroundColor = .clear
                    }
                    return cellView
            }
        } 

I've tried to change size constraints in many ways, to no avail.

For instance, I changed Layout to Autoresising :

I tried to change TableCellView size to 170 vs 144:

Or increase tableColum Width.

I have looked at what other object in the NSTableView hierarchy should be coloured without success.

Nothing works.

What am I missing ?