The implicit effective metric assembles the stretch stiffness from
world-space edge vectors, but a flex vertex body's slide dofs are
expressed in its parent body's frame. When that frame is rotated the
assembled operator is therefore not the Jacobian of the passive stretch
force, which mj_flexPassiveStretch already maps into the dof frame with
xmat^T. The metric is then inconsistent with the force it linearizes:
implicit integration loses its stability guarantee, and models that the
same flex handles comfortably in an unrotated frame diverge.
Apply the matching change of basis in both places that build or apply the
stretch stiffness: mjd_flexStretch_mul rotates the input dof vector into
world and the scattered result back, and mjd_flexStiff_assemble sandwiches
each 3x3 block as R_bi^T * blk * R_bj. Both are no-ops when the parent is
unrotated. Bending needs no change: its blocks are isotropic, and
R^T (q I) R = q I.
This completes the fix in fe9dc584, which covered the passive force paths
and the interp (trilinear) derivative but not the standard stretch one.
On a mesh flex inside a body with a 90-degree rotation, the metric's
directional agreement with the force Jacobian goes from cos = 0.57 to
cos = 1.0, and a hanging sheet that previously reached 176% strain
settles at 0.87%.
Alessio
55b43414dd