Fixing issue with tasks being laid out with the wrong bounds.

- When hovering over a drop target, we update the stack bounds, which 
  changes the task rect.  If we happen to pick up new tasks and measure
  them during this point, they will be measured with the wrong rect, and
  once you stop dragging over a drop target, they will not have the 
  right bounds.

Change-Id: I6711152878a4afd48518e01477fd793a4c132648