Scratch

It uses a different method for each edge. For example, the code for one of the side edges is

((((direction) - 90) * (-1)) + (270))

Taken from the Scratch Wiki:

when gf clicked
go to x: (0) y: (0)
point in direction (pick random (-179) to (180))
set [Move Speed v] to [0]
forever
move (Move Speed) steps
if <touching [edge v]?>
change x by (([sin v] of (direction)) * ((0) - (Move Speed)))
if <touching [edge v]?>
point in direction ((180) - (direction))
else
point in direction ((0) - (direction))
end
change x by (([sin v] of (direction)) * (Move Speed))
end