The normal behaviour of PRIMARY selection in linux (middle mouse button) is that it remains until a new selection is made. Some editors, like Atom, honour this behaviour. In Geany, after selecting, a left mouse click anywhere inside the editor erases the PRIMARY selection: You cannot paste it any more.
I think that keeping the PRIMARY selection is more natural: you are not forced to paste immediately after selection! Besides, because it is less natural, I frequently end pasting in the wrong place.
Oddly, if you select and middle-button paste imediately, now PRIMARY selection becomes permanent: you can left-click as many times as you want (and middle-mouse paste as many times as you want), and middle-button keeps the PRIMARY selection.