If you would like to implement it now here’s the basic logic:
What you will need is an additional variable to store the selected slot, and have it saved in InventoryManager when the OnPointerClick is triggered in the InventorySlot. (If the slot has something)
Then on the same OnPointerClick function is triggered in the InventorySlot and it is empty, you can write a function such that it transfers the selected slot entirely as a stack on left click, and only 1 on right click.
To implement this you’ll need to:
Add the selection tracking variables to InventoryManager
Create a UI element for the selected slot highlight