R1:0e98e33b3342

When dragging stuff around, compensate for changes in scroll position

Summary:
Ref T5240. This fixes dragging stuff and using the mousewheel or keyboard to scroll the window during the drag.

(It does not fix "dragging near the edge of the container should scroll it" yet.)

Test Plan: Dragged stuff around on task list and workboards in Safari, Firefox and Chrome. Used mousewheel and shift + mousewheel to scroll the document and containers during drag. Object remained under the cursor.

Reviewers: btrahan…