I'm excited to see what's in store for the new inventory system. Cursed cannonballs are only the beginning. However, I agree with many who have issues that this change has presented. Here is my [proposed] solution[s].
hold X - Pickup container/(chest, crate, etc)
hold Y - Open Container (new Inventory UI)
tap X - take resource from container
tap Y - store resource in container
• When tapping X to take resources, if there are multiple types of items (e.g. bananas & planks), they should be prioritized to be taken in this order: bananas, planks, cannonballs, cursed cannonballs.
• If no resource is equipped when Y is tapped, draw weapon.
• If a resource is equipped when Y is tapped, but no container is near, draw weapon.
• If taking items within the inventory UI, any items that are 0x should disappear and the next item be selected. Don't leave a 0x ghost behind.
• Tapping X/Y to take/store resources should trigger on button release, rather than button press. Tapping X or Y should have an auditory cue on both the button press and release, each being distinct, to help the player understand how the operation works.
• The wait time for holding Y to trigger opening the container should be the same as holding X to trigger picking up the container.
• All barrel/container labels should show how many items are in the barrel. (like the used to be before 1.2.4)
• If there are no resources that can be taken, the barrel label should be greyed out.
I think this will be a comfortable compromise between the new system (that will be necessary for new additions to the game) and the old system that was quick, responsive, and very fluid to use in the heat of battle (PVP & PVE).
Edit:
Another possibility would be to display "Hold Y to Open Barrel" option only when the player has nothing in hand, otherwise if they are holding, say a banana, "Tap Y to Store Banana" would display instead. In other words, you would have to drop/stow whatever is being held in order to see the option. That would help with the visual clutter.
Edit2:
Instead of icons showing the different type, the mystery could be preserved by just indicating (with the ellipsis) that there are more than just what's "on the surface" to take off the top of the barrel. You could still take items, one by one, that can be taken, but if you want to look inside to see everything, you still can.