Fast path in mj_flexPassiveInterp, mj_flexPassiveBendInterp, and
mj_flexPassiveStretch assumed body slide joints are world-aligned
(J = I). When parent body has a non-identity quaternion, joint axes
are rotated (J = R_body), causing wrong force mapping and instability
(NaN/Inf in QACC).
Fix: project world-frame forces onto body local frame via
mju_mulMatTVec3(R_body^T, force) before adding to qfrc_spring/damper.
Also fix the derivative paths:
- mjd_flexInterp_kernel fast path: R^T * K_rot * R * vec
- mjd_flexStiff_assemble (CSR): R_bi^T * K_rot_block * R_bj
The CSR-assembled stiffness matrix is the actual path used by the
implicit CG solver (mj_flexCG gate). The test uses solver="CG" to
activate this path; without it, flex stiffness is integrated
explicitly and no derivative fix can help.
Ported from GitHub PR https://github.com/google-deepmind/mujoco/pull/3379
Original author: Devansh (https://github.com/devansh0703)
Fixes https://github.com/google-deepmind/mujoco/issues/3364
PiperOrigin-RevId: 952789284
Change-Id: If924f7160dd16c0cc88170d80e6e605da0fe2e04
Alessio Quaglino
fe9dc58477