Vanilla:

Spinnery:

Also, WSlot.addArray places adjacent slots with like 1 pixel between them, vs. vanilla which places them right next to eachother.
Also also WSlot.addPlayerInventory doesn't add the "Inventory" label

Will fix.

The item offset has been fixed by fixating WSlots to an int grid, instead of a float-based grid like the rest of Spinnery; thanks for the report!

I was however unable to reproduce the addArray offset issue myself; and addPlayerInventory will not add the label because the developer may not want the label, eg. in a screen similar to the creative screen.

Mouse over the border between 2 slots in Spinnery and you can see that you don't select any slots.
Do the same thing in a vanilla GUI and you should see that you select the 2nd slot.

The addPlayerInventory quirk is understandable, though. Guess I'll just make a utility method that adds the label.