VS2015: Dragdrop fires after treeview selection, even if left mousebutton not down

Question

I recently implemented drag-drop from a treeview to desktop in my application (WinForms). There are also handlers for dragdrop events on the treeview itself, as well as a listview that is already present on the page (that functionality existed before I allowed for filedrop onto the desktop). 

The problem I'm having is that if, in the treeview, you click on a node, then release the left mousebutton, the drag operation seems to still occur. If I move the mouse (left button up) over the rest of the treeview, it tries to Drop the clicked node onto the node hovered over. 

Why isn't it looking for whether the mouse button is depressed or not? Is there some way to tell it "hey, if you're being hovered over, and the mousebutton is not depressed, don't drop?"

Thanks for any info.