You can drag objects from one window to another even if the destination window is not visible. For example, the destination window may be covered by other windows. To drag to a hidden window, you use the Task Bar
Dragging objects to a hidden window
1 Drag the object to the Task Bar button that represents the hidden destination window.
2 You cannot drop the object onto the Task Bar button, but if you briefly pause the object over the Task Bar button, the hidden window will come to the foreground. You can then drop the object onto the destination window.