Copy Location constraint from bendy bone shows crazy jiggle behaviour in linked proxy rig.
Open, ConfirmedPublic

Description

System Information
Windows 10, GeForce GTX 950M

Blender Version
Broken: 2.78b, 12b62b5

Short description of error
See the files:
stretch.blend contains a rig with a bendy bone "A", its custom handle references "tangent_A1" and "tangent_A2" (used to control and animate it), and a bone "Bone" with Copy Location constraint set to the middle of the bendy bone.
linked.blend contains the same rig linked as proxy. Both tangent bones are animated.
"Bone", instead of sticking to the middle of "A", jiggles around like crazy (no proper update) when scrubbing through the timeline.

The problem appears only when the bone layers are protected.
When I append the action back to the original rig file, everything works fine.

Exact steps for others to reproduce the error
Download both files into the same folder, open link.blend and scrub through the timeline.


Details

Type
Bug

The problem persists in 2.78c and today's dayly build 4d0d1b5

This is evaluation issue… Some notes:

  • Looks like if the differential movement between two frames was applied twice. That’s why constrained bone goes back to its place after few frames without any movement.
  • With old depsgraph, enabling Extra Data Update for the proxy and the instancing group fixes the issue.

Maybe @Sergey Sharybin (sergey) or @Joshua Leung (aligorith) will have ideas, before I dive too deep in that nightmare?