Optimistic update on dragging

[itsjoeoui]

Note: need to also implement fracdex on the frontend such that we can also compute the optimistic position value directly on the frontend. Otherwise it will be broken for multiple drags in a row if we just use the index value.

This may be a bit hard for a "good first issue", which I may do it myself hahaha

[linear]

FIJ-56 Optimistic update on dragging