Coherent nonlinear stationary state is studied in the frequency and spatial domain of electromagnetic lower-hybrid waves in a beam-plasma configuration that is subjected to the oblique linear Buneman instability. In the presence of magnetic shear, a solution is found in the form of a nonlinear chain of magnetic islands. Such equilibrium may represent the saturated state of fast collisionless magnetic reconnection that is reached after the reconnection has ended. It is demonstrated that the topological constraints that hamper the reconnection of magnetic field lines in the fluid representation are efficiently removed by the nonlinear currents and space charges of trapped electrons. Two distinct trapping mechanisms are distinguished. Besides the electrostatic trapping of particles in the self-consistent minima of potential energy, a different type of magnetic trapping is identified that is responsible for the creation of thin current sheets at the separatrices between open and closed magnetic field lines. Combined effects of the resulting two types of nonlinear phase-space vortices provide for a topological transformation of a sheared magnetic field into a single chain of magnetic islands coupled with a double chain of hydrodynamic vortices.