= Solution
The relevant competition is between energy reduction and <magnetic flux freezing>. Ideal evolution transports the entire connectivity of the <magnetic field lines>, including the separation and linkage of flux regions. <Viscous dissipation> can remove <kinetic energy> but cannot permit <magnetic reconnection> between such regions. As each region relaxes toward lower <magnetic energy>, neighboring flux regions can approach with different limiting tangential fields. A smooth transition can become increasingly thin; its field remains bounded while its <curl>, and therefore its <electric current density>, becomes large.
The limiting object is a <current sheet>. If $\mathbf n$ points from its minus to its plus side, its surface <electric current density> is
$$
\boxed{\mathbf K=\frac1{\mu_0}\mathbf n\times(\mathbf B_+-\mathbf B_-).}
$$
For a tangential discontinuity, $\mathbf B_\pm\cdot\mathbf n=0$. The normal balance in <magnetostatic equilibrium> requires
$$
\boxed{\left[p+\frac{|\mathbf B|^2}{2\mu_0}\right]_-^+=0.}
$$
Thus a finite jump of tangential field can coexist with equilibrium: the jump of <magnetic pressure> is balanced by the ordinary <pressure> jump.
This need not happen for every initial field; a field already in smooth <magnetostatic equilibrium> is an immediate exception. The expectation concerns topology-constrained relaxation for which a smooth limiting arrangement is obstructed. Current-sheet formation is consistent with the relaxation framework developed in https://www.damtp.cam.ac.uk/user/hkm2/PDFs/Moffatt_1985_JFM_159_359.pdf[Moffatt's primary analysis]. Retaining complete frozen connectivity is essential; minimizing energy subject only to the value of total <magnetic helicity> would generally permit field rearrangements forbidden by this evolution.
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