Two of the remaining issues would be nice to have fixed regardless:
[x] Starting from 1.13, dropping an item with the drop button with no inventory open does not update the item count on the client. Requires tracking of the current slot + hand item
[ ] For 1.16.5 clients, some dragging action can cause an accidental collect all click: https://streamable.com/347v07
Most issues were fixed by https://github.com/ViaVersion/ViaBackwards/commit/30a3a4fb89c01cd3237a9f16fcb8ce226ac3b85a and going beyond would require emulating/copy-pasting a whole lot of Vanilla inventory handling as well as full inventory tracking, so that is probably not going to happen.
Two of the remaining issues would be nice to have fixed regardless: