Drag superheroes around the field

This task can help you to check understanding of several aspects of Drag'n'Drop and DOM.

Make all elements with class draggable -- draggable. Like a ball in the chapter.

Requirements:

Use event delegation to track drag start: a single event handler on document for mousedown.
If elements are dragged to top/bottom window edges -- the page scrolls up/down to allow further dragging.
There is no horizontal scroll (this makes the task a bit simpler, adding it is easy).
Draggable elements or their parts should never leave the window, even after swift mouse moves.

The demo is too big to fit it here, so here's the link.

[demo src="solution"]