Keyboard shortcuts changed in 3.9.1
It's a little thing but a PITA if you need it. With 3.9.1 the keyboard shortcuts to view next and view previous in the image viewer changed from: 3.6.1 - Space Bar or Page Down for next and Backspace or Page Up for previous. Now on 3.9.1 it's only page up and page down to navigate forward and backward. This is problematic on limited keyboards such as laptops and the Apple wireless aluminum to name a few which don't have dedicated page up and down buttons. These functions are now also different in presentation mode.
Sorry - am closing the issue as I found the shortcuts settings and have corrected it.
Edited by Tony-329