Carpet mod and essential client allow stacking shulker boxes inside inventories by a player. In order to not break default behavior I think stacking boxes should be a setting in the mod.

this can only be done server side
this mod is client side only

and oversized stacks are "supposed" to a degree and are either left untouched ore moved at the end if possible

https://modrinth.com/mod/essentialclient

It allows stacking shulkers in player inventory if carpet rule is enabled on the server

So with proposed option enabled when sorting inventory mod will attempt to stack shulker box items. On most servers it won't work so its supposed to be disabled by default
@blackd

unfortunately the whole point of this mod is to not require any server side component or non vanilla communication

so this will not be added