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Orthogonalize the constraint target's matrix before decomposing it into Euler angles. This removes sheer, and is actually a requirement for correct decomposition. It's conceivable that someone has used the incorrect behaviour in a rig. As the shear caused unpredictable flipping of the constrained object, this is unlikely. Reviewed By: angavrilov, sybren Differential Revision: https://developer.blender.org/D8915