If the target node is not visible when you start the drag operation, you can do the following navigation while you have the left mouse button depressed during the drag:
Scroll Up or Down
Move the mouse cursor near the top or bottom of the tree window. You may have to move slightly outside the border to start the tree scrolling. The tree will continue scrolling until the top or bottom, or until you move the mouse back into the window.
Open Nodes with Children
Hold the mouse over the node for a second or so, and the node will open. The node must be highlighted for this to work.