To solve issue above [weird usage of ItemCount], made update
2018.06.08
+ ATListbox: added props VirtualMode, VirtualItemCount
If VirtualMode=false, listbox uses item count from Items prop.
If true, listbox uses item count from VirtualItemCount prop.
+ ATListbox: prop ItemCount gets count for all modes, and is read-only
Listbox demo shows how to use them. it adds 4 real items (to Items prop) and 21 virtual items (setting VirtualItemCount). and allows to switch VirtualMode: show real items or virtual items.