Add clickable function, i.e. click and release methods, for Slot element. Try out a test snippet.

Shoes.app do
  f = flow width: 0.4, height: 0.5 do
    background green
  end
  s = stack width: 0.6, height: 0.3 do
    background yellow
  end
  f.click{|b, l, t| alert "%d %d %d" % [b, l, t]}
  s.release{|b, l, t| alert "%d %d %d" % [b, l, t]}
end