This system can create an inventory where you can place any items anywhere, as well as modify. Something similar to minecraft inventory. This systems also allows for overfill slots so you can have multiple slots with the same items.

WASD to move

Arrow Keys, clicking the slot or typing the slot number will select the active slot. Even double digits work!

Use activate slot with E

H for Health, or highlight apple and press E

Spacebar for Cheese, or highlight cheese and press E

Merge/Move slots by holding M and selecting the active slot. You can also click and drag the slot.

Split can be activated by pressing X or by right click. While split is active, press Z or click on the highlighted slot to split the item off by 1 into the new slot. While split is active, press X or right click again to split the quantity in half into the new slot. This will also deactivate Split.

This Inventory does not allow limits per item, only a global for all slots. You will have to make per item limits outside of this inventory system.