A another idea based upon the earlier post.
The Problem
When dragging an item, the target is underneath (hence blocked) by a window, pallet, toolbar, etc.
A Solution
If a window can accept the draggable and the window below it can also accept it, then the top window can display a Hide button that is triggered not with clicking but with the motion of the mouse. See DONTCLICK.IT for example of buttons.
Before dragging the triangle, the second window is over top of where the desired drop location for the triangle on the first window : 
While dragging the triangle over the second window, the window displays a button to hide itself: 
By moving the cursor over top of the Hide button, the confirmation button for the button is next to the Hide button: 
Then to hide the second window the user moves the cursor from the Hide button to the confirmation button: 
Finally the second window is hidden (after the draggable is dropped, the second window would show back up again):