The Problem
When dragging an item, the target is underneath (hence blocked) by a window, pallet, toolbar, etc.
A Solution
Any window (and such) that cannot accept (meaning your cursor changes to a circle with the cross through it when over) the draggable item will get out of the way. They can shrink, resize, move, or fade to get out of the way. Better yet, only do so if a possible target is beneath it.
Before dragging the triangle, the pallet is full size and over top of the canvas: 
While dragging the triangle, the pallet shrinks in sizes and dulls because the pallet does not accept the triangle shape: 