The top rail of the mirror frame projects slightly beyond the vertical stiles. These end-grain surfaces are chamfered, softened and finished prior to assembly, since it's much easier to control the process before the parts are assembled.
Corner #1 glued-up. The first two corners are done one at a time. This allows plenty of time for squaring the joint before the glue begins to set.
Of course, the last two joints are glued at the same time to complete the frame assembly.