A complete support of anim data edition in library overrides data-blocks is a rather complex and heavy task. Here is a proposed split of it into milestones. Please note that not all of those milestones will get a defined release targets yet.
Milestone 0 - Already in master
- Add or replace the active action of an override data-block.
Milestone 1 - Minimal Viable Product - 2.92
- Support adding new NLA tracks to overrides that already have animdata from their linked reference data-block, and make those new tracks fully editable.
- Enable overrides of some properties of existing animdata (like show/hide for NLA strips, tracks, fcurves, settings of drivers?).
Milestone 2 - Potential Extensions - ???
Those will require T82160: LibOverrides - Refactor how diffing of RNA collections is handled, and extend diffing possibilities. to be tackled first.
- Support editing of existing NLA tracks from the linked reference data-block, including adding/removing/re-ordering existing strips? Not sure how useful that would be in practice though.
- Support adding/replacing FCurves in overrides of Actions and/or animdata (for drivers)? Same remarks as above.
Operating system: Windows-10-10.0.17763 64 Bits
Graphics card: GeForce GTX 980/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 432.00
Broken: version: 2.81 (sub 16), branch: master, commit date: 2019-11-20 14:27, hash: rB26bd5ebd42e3
Short description of error
When animating armature override *that contains drivers*, actions added to NLA tracks are lost after saving/reloading blender file.
Exact steps for others to reproduce the error
1 - Load testCubeDriverNLA.blend (contains link to cubeRigDrivers.blend)
2 - Use "push down action" from the NLA editor to add current "ArmatureAction" action to a new track
3 - Save the file
4 - Reload the file: the NLA track is lost