Take a look on GitHub - dgraph-io/graphoverflow: Run the entire StackOverflow on Dgraph. Work in progress.
Drag and drop operations are an application feature (not Dgraph). With Dgraph you could create a node for each intem of the list and use sort functions in queries.
I would recommend reading these articles:
Blog Post: https://blog.dgraph.io/post/building-graphoverflow/
Blog Post2: https://blog.dgraph.io/post/sql-vs-dgraph/
Sorting: https://docs.dgraph.io/query-language/#sorting
On performance, hashing is the best indexing for this.