The impact on electron dynamics of the nonstationarity and nonuniformity of a quasi-perpendicular supercritical collisionless planar shock is analyzed by means of a two-dimensional full-particle electromagnetic simulation code. Trajectories of preselected self-consistent electrons (in contrast with the test particles approach) have been analyzed in order to determine the reflection conditions. Four different classes of reflected electrons have been identified according to the time these spent within the shock front itself before being reinjected into the upstream region. However, the shock front is revealed to be strongly nonstationary and nonuniform (front rippling). Different sources of shock front nonstationarity are shown to take place over different time and spatial scales. The longer time scale is due to the shock front reformation associated to the dynamics of reflected ions. The smaller time scale is due to the propagation of front rippling along the shock front. One key result is that electrons hitting the shock front are not reflected uniformly in time; instead, bursts of energetic reflected electrons are formed by the shock front reformation. In addition, electrons are not reflected uniformly in space, but packs of reflected electrons are formed along the rippled shock front. Comparison of two-dimensional with one-dimensional simulation results evidences that nonuniformity (rippling) of the shock front contributes to diffuse electron bursty patterns both in real and velocities space. Persistent (one-dimensional) and more diffuse (two-dimensional) electron bursts have different impacts on upstream wave emission, which are also discussed.