Yes but you could work around it. Check out this example:

Code:
dialog t {
  title ""
  size -1 -1 300 300
  option dbu
  list 1, 0 0 300 270, vsbar
  button "clear", 2, 270 270 20 20
  button "add item", 3, 240 270 20 20
}
on *:dialog:t:mouse:*:{
  if ($did(2).enabled) && (!$did(1).lines) did -b $dname 2
  elseif ($did(1).lines) && (!$did(2).enabled) did -e $dname 2
}
on *:dialog:t:sclick:2: did -r $dname 1
on *:dialog:t:sclick:3: did -a $dname 1 test