PlanarConstraint's point-in-plane residual used a body-attached normal
(z_j) that rotates with body_j. When combined with CylindricalConstraint,
which allows rotation about the shared axis, the plane normal tilts
during Newton iteration. This allows the body to drift along the cylinder
axis while technically satisfying the rotated distance residual.
Fix: snapshot the world-frame normal at system-build time (as Const
nodes) and use it in the distance residual. The cross-product residuals
still use body-attached normals to enforce parallelism. Only the
distance residual needs the fixed reference direction.
Works for any offset value: (p_i - p_j) · z_ref - offset = 0.
forbes-0023
000f54adaa