Window moves underneath the cursor when starting a drag with representative()

When I use `representative()` to make an element draggable by just its child, the window jumps underneath the cursor such that the window’s top-left corner is under the cursor.

Demo: http://jsfiddle.net/eveLzaaw/2/

Start by reproducing the behavior in the linked jsfiddle and then trace the representative() drag implementation. Compare the cursor position with the window position when the drag begins. Done means dragging through a child preserves the initial cursor-to-window offset instead of moving the window's top-left corner under the cursor.

Written by the indexing model from the issue text.