Blender Git Loki

Action Editor: Go to Next/Previous Animation Layer

With this feature, it is now possible to quickly switch between different actions
stacked/stashed on top of each other in the NLA Stack without having to go to the
NLA Editor and doing a tab-select-tab dance, thus saving quite a few clicks. It
was specifically designed with Game Animation / Action Library workflows in mind,
but also helps layered animation workflows.

Usage:
Simply click on the up/down arrow buttons (between the action datablock selector
and the pushdown/stash buttons) to go to the action in the NLA Track above/below
the NLA Strip being whose action is being tweaked in the Action Editor.

Notes:
- These still work when you're not editing the action used by a NLA Strip.
If you're just animating a new action normally, it is possible to use the "down arrow"
to temporarily jump down to the previous action without losing the new action you're
working on, and then use the "up arrow" to get back to it once you're done checking
the other action(s).

- If there are multiple actions/strips on the same layer/track, then only the one
closest to the current frame will be used.