R1:f02aed10e769

Fix three issues with scrolling while dragging items (e.g., on workboards)

Summary:
Fixes T5979. There are three issues here:

- We cache document positions when you pick an item up, but don't recalculate them after you scroll, so they get out of date. Dirty the cache when the user scrolls.
- When we rebuild the cache during a drag (previously, this never happened), the position of the object you're dragging is computed wrong (since it has been moved to be under the cursor). Adjust the effective…