When you press the keybind (K by default), it moves all items from your inventory into nearby containers that already contain those items.
The range in which to check for nearby chests can be configured in config/quickstack-common.toml. It is 16 by default. Note that all containers within the range are included, even ones blocked by other blocks.
Installing Inventory Profiles Next is highly recommended, that way you can lock your hotbar and other useful items which prevents them from being quick-stacked as well as dropped, shift-clicked, etc - press Alt and click on slots you want to lock, very useful even without quickstack.
Please note that this MOD has been entirely made by Google Jules AI. However I am using it in my heavily modded playthrough and it works, so enjoy. I personally can't live with quick stack and it is a crime that no one has made it for 1.21.1 NeoForge.
https://github.com/italian-brainrot/jules-quickstack-1.21.1-neoforge
Как поиграть с друзьями с модом jules-quickstack?
Мод jules-quickstack куда интереснее в компании: поднимите сервер Майнкрафт с уже установленным модом, позовите друзей и играйте по своим правилам. Хостинг Майнкрафт для мода jules-quickstack будет готов за пару минут - BungeeHost сделает всё за вас.