Each track of the Oberon system consists of a number of viewers. The filler is the top-most viewer in a track and covers the remainding part of the track the viewers do not cover. The set of tracks form the root objects of the display space.
Tracks can overlay each other.
Overlayed tracks and viewers do not receive message broadcasts in the display space. Before being overlayed, the contents of a track receive a Display. The logical display increases from X coordinate 0 onwards through multiple physical displays. Oberon uses a single coordinate system to address all the different displays. Note that many Oberon systems restrict the size of the display to the size of the host window.