The bookcases in Macaw's Furniture combine visually when stacked with the same facing, but the inventories remain separate. This both makes it a bit awkward to target a specific bookcase, and doesn't match the expectation from vanilla chests that the inventories combine when the models combine.
I think the ideal unified inventory would:
show all the inventories for the column of aligned bookshelves stacked in the same vertical order as the bookshelves,
have a small line between each shelf's inventory (maybe the line is the same color as the bookshelf's wood color, but leave the background grey for visibility of the contents)
have a scrollbar if there are too many combined inventories for the screen height.
when opening a combined bookshelf inventory of a stack large enough to have a scrollbar, the clicked shelf's inventory is scrolled into view on open.
The bookcases in Macaw's Furniture combine visually when stacked with the same facing, but the inventories remain separate. This both makes it a bit awkward to target a specific bookcase, and doesn't match the expectation from vanilla chests that the inventories combine when the models combine.
I think the ideal unified inventory would: