@richardl
As I see it, showing both forward and backward entries at the same time is a smart idea if your interface requires having a single button for this feature. Otherwise, it doesn't make much sense because one would never click on the backward navigation button to pick a forward entry. Having to see the forward entries first to decide which backward entry to pick is not a common case either (and vicecersa). Thus, in most cases you'd just have to deal with the drawback of limiting the amount of forward-only or backward-only entries that can be shown, which in addition is counter-productive if you want to roll back a lot of entries.

Hi there,
I just wanted to point out that Falkon, a KDE application, already implements this with a kind of buttons very similar (if not the same) to the ones Dolphin uses: