The bug
A fake inventory is rendered in the background of the recipe book if it is rendered in front of the inventory because the window is not wide enough. This fake inventory is not rendering the items the player actually has in the respective inventory slots additionally if the player had an item picked up with they cursor when opening the recipe book the item is not displayed anymore but is still there which can be seen when clicking outside the GUI causing the item to be dropped.
How to reproduce
Place items in the first and last slot of your hotbar
Open the recipe book
Resize the window to have the recipe book being display above the inventory
→ The fake inventory in the background displays no items in the first and last hotbar slot
Linked issues
is duplicated by 10
relates to 2
Attachments
Comments 5
Can confirm in 21w03a.
Can confirm in 21w11a.
Can reproduce in 23w41a.

This bug is still present in 1.20.2. When the window size is, for example, 1280x1024 (full screen mode on my display), the inventory becomes inaccessible because the recipe book is placed over inventory. For a workaround the window can be resized, but this becomes tedious quickly.
Can confirm in 20w48a.