I can confirm this, but I'm not sure if this is intentional or not.
dialog xyz {
size -1 -1 420 320
edit "", 1, 5 5 400 300, edit vsbar multi
}
1. /dialog xyz xyz
2. //did -a xyz 1 $str(x,4000) (repeat many times)
3. //echo -a $calc($did(xyz,1).lines * $did(xyz,1,1).len)
4. Repeat from step 2 a few times. Result maxes at 30012