Not possible to copy using entry popup
The changeset 709fa6e42bd8 from #2230 (closed) introduce a PositionManager that reset the position on focus out but this prevent to use the entry popup as it triggers the focus-out event when it pops and so no text is select.
I think we should stick as close as possible to the GTK+ default behaviour and so I propose to just set the position to 0 when calling set_text like that the beginning of the entry will be visible the first time and after that it is up to use the user to manage the cursor position.