I'm using a Unity UI for my inventory that has a total of 14 slots.
Each inventory element and button is linked correctly in the AC menu and the Unity UI's Grid is set to centered, with no padding at the left or right sides.
The issue what appears to happen is that when new inventory is added, it places it at the front of the 14 slots, and even though the slots are empty and the menu is set to disable objects when empty (I've tried 'Clear Content' too) it's still counting those empty slots as if they're a part of the inventory, so the menu appears left-aligned.
See a screenshot below:

Here's the inventory also added into the scene so you can see how it's aligning them to the left and not removing the other slots:

All slots have no images applied to their buttons' element and each button is identical.
Here's the UI:






And here's the AC menu:

As I mentioned previously, I've tried setting the 'When slot is empty' to Clear Content too but it doesn't change anything.
Even though I'm making a game that supports multiple aspect ratios, I've tried it with everything based at 16:9 but it's still the same. Even in other aspect ratios the menu is technically centered but the empty buttons are still being counted, despite that they shouldn't be counted/included at all.
I'm sorry for all of the images but I figured it's best to have a clear view of what I'm trying to explain.
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I'm afraid I'm not quite clear. Is the issue to do with item's ordering, or the alignment of the whole box, in that they should always be centre-aligned?
If you want it to be centred, you'll need to set it When slot is empty to Disable Object, but other than that it's down to Unity UI's component usage and not AC.
To have the parent "Items" object shrink itself according to what children are enabled, you'd likely need to rely on a Content Size Fitter component.
Basically, shouldn't the visible inventory items be centered and the empty slots removed, thus centering the available slots? What's happening is the items are staying to the left as if the empty slots are still physically in the menu. I hope that makes sense.
I'll play around with the Content Size Fitter and see if that can sort it.
Basically, the empty button slot aren't being disabled, despite having 'Disable Object' selected, so the centered inventory is always off-centre.
Or is this by design and I'm confused how this works?
Disable Object should behave as you expect - disable the Button's GameObject. Is something else e.g. an Animator controlling it?
Even with the Animator component disabled it's still the same. I've recreated the menu from scratch and it's still the same. Very odd behaviour.
What's your AC version?
PM it to me, along with the Menu Manager, and I'll take a look.
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Ah - just check Limit maximum scroll? to prevent empty slots from being shown.
I'm sure I tried that before, but obviously not as it seems to work. It doesn't seem QUITE central but it's better than it was so I'll take it. Thanks, Chris!