The bug
The translucent white effect displayed in the inventory when the cursor is on top of an item slot cannot have its texture changed, and seems as though it may be hardcoded.
How to fix
Introduce a texture file for this effect (a translucent white square should suffice).
Linked issues
relates to 28
MC-278413
Chat background cannot be retextured
Open
MC-264571
Both standing and wall sign GUI backgrounds cannot be retextured independantly from the block
Resolved
MC-268805
panorama_overlay.png no longer works
Resolved
MC-268927
The dark background in inventory screens cannot be changed via resource packs
Resolved
MC-269079
Death screen background cannot be retextured
Resolved
23 more links
Also cannot be retextured in version 1.20.5 pre-release 3.