I would like to add a button next to the disclosure button like in Mail.app.

And I don't want to do it like this in Photos.app.

I had a look at the answers in this question but they are about customising existing disclosure buttons which I do not wish to modify. I just want to add a button next to it and not for every group item. I believe it has to be done programmatically.

I tried adding a button on storyboard and place it on the right of the NSTableCellView and the disclosure button overlap it. So I think it should not be done on storyboard.