Also, it may certainly seem odd if you don't know how to use it. And yes maybe still if you know how.
All features have instructions via the "..." button next to where you select the feature in the ThorUI AU.
The inventory window DOES have all equipment slots, they are just hidden so that you may leave the inventory window up while you play for quick right click cast items and so forth.
Basically the big button on the top left will expand the window if you were to click it.