So, I figured out quickly that you could only take 4 items to the next floor, and there was a top row in the inventory that was kept.

The problem is, and I keep on doing this, that right before picking up the crystal, I go over to a shop, equip the items I want, and sell everything so I've got only 1 row of 4 items left that I want to keep. Then I go, pick up the crystal, go to the next level, and find half of my items are gone! Because selling items in the shop *shows* my 4 remaining items on a single row, I keep assuming they're going to be kept, and forget to look in the inventory to see what seemingly arbitrary set of them (after I did my equip/sell shuffle) it's decided it wants me to keep, and rearrange my only 4 items into those 4 slots.

It would be great if, in the case of empty backpack slots, it always kept the next items. Alternatively, if the shop inventory screen reflected which were in the backpack (two separate rows or some other indicator), though that would make that screen a bit ungainly. Alternatively, perhaps the simplest, the game could, when leaving a shop, move items into the backpack slots automatically, so that opening the inventory immediately after leaving a shop always presents the exact same rows of ordered items you saw within the shop.