no "back" button when in single photo view

Binary package hint: shotwell

after double clicking an image to deal with it in single view, i would expect to have a mouse way of getting back to the previous view, position, pan and scale at once.
1) a simple "back" button would do the trick, since changing from gallery to single view is like opening an extra page.
2) Alternatively, the thumb, upon double click, could scale in over the gallery, eventually replacing the gallery view. In that case there should be a "thumb view" button to take me back to the previous view sice, since scaling in is reversed by scaling out in the user's natural mental model.