Deleting a node should not move the focus to the root node
When I delete a transformation or proof node with the Del shortcut, the focus is moved all the way to the root node, making the process of deleting several contiguous nodes (or deleting a node mid-proof) painful. It would be much better if the focus instead moved to a closer neighbour of the deleted node, such as a sibling (if available) or its father (if not).