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- We cache document positions when you pick an item up, but don't recalculate them after you scroll, so they get out of date.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 position of the object you've picked up to put it back in the right place in the list.
- When you fiddle around at the bottom of a column you can get jumpy redraws as the height adjusts. Put min-height on the container during a drag to prevent this.