The TVM_GETVISIBLECOUNT message obtains the number of items that can be fully visible in the client window of a tree-view control. You can send this message explicitly, or by using the TreeView_GetVisibleCount macro.
TVM_GETVISIBLECOUNT wParam = 0; lParam = 0;
Returns the number of items that can be fully visible in the client window of the tree-view control.
The TVM_GETVISIBLECOUNT message returns the number of items that can be fully visible, which may be greater than the number of items in the control. The control calculates this value by dividing the height of the client window by the height of an item.
Note that the return value is the number of items that can be fully visible. If you can see all of 20 items, and part of one more item, the return value is 20, not 21.