R1:fce010982282

When dragging nodes, clone them

Summary:
Ref T5240. Currently, when dragging nodes, we leave them where they are in the document and apply "position: relative;" so we can move them around on screen.

- Pros: All the CSS still works.
- Cons: Can't drag them outside the nearest containing element with "overflow: hidden;", many subtle positioning bugs with scrollable containers.

Instead, this diff leaves the thing we're dragging exactly where it is, clones it, and drags the clone instead.

- Pros: You…