The bug

24w44a has finally split off the slot background outline textures for rideable mob inventories. For rendering the slot shape itself, the game now uses slot.png, which means it is no longer possible to change the slot texture in these mobs' inventories independently of each other, or the slots in the statistics screen or superflat customization screen.

How to reproduce

A resource pack that changes slot.png is attached.

1. Download and apply the attached resource pack
2. Check the statistics screen
3. Check the superflat customization screen
4. Check a rideable mob's inventory

Expected results

The slots in the rideable mob's inventory wouldn't have their texture changed by this pack.

Actual results

They do.

How to fix

The solution that makes the most sense would be to give each rideable mob its own inventory texture, since no other inventory in the game uses slot.png for rendering slots. I've attached example textures for each rideable mob, which Mojang have full permission to use. The game would need to render less graphics using this solution, and this would permit the same level of customization as before.