[Translated using translation software] 

[Update: I retested in version 1.21.4 Pre-Release 1, and the empty slot outlines in the brewing stand and mount GUIs now appear between slot_highlight_back.png and slot_highlight_front.png. However, the highlight effect for empty slot outlines in the smithing table and loom GUIs seems unchanged.]

After the update in version 24w39a, which states "Updated slot highlight in the UI so Item texture has better visibility," differences in the outline highlight effect of empty slots occur when hovering over different GUI slots. This is caused by the different layering order between the slot outlines in various GUIs and the game files \assets\minecraft\textures\gui\sprites\container\slot_highlight_back.png and \assets\minecraft\textures\gui\sprites\container\slot_highlight_front.png.

Through testing, it was found that the empty slot outlines for armor slots and the off-hand slot in the survival inventory, as well as the lapis lazuli slot in the enchantment table, appear above slot_highlight_back.png and below slot_highlight_front.png. Meanwhile, the empty slot outlines in the brewing stand, smithing table, loom, and mount GUI appear below both slot_highlight_back.png and slot_highlight_front.png. (Possibly because the empty slot outline sprite has a gray background or is not separate, and thus had to be designed this way; a pinned comment explains some related issues.)

Attached is a comparison of the empty slot outline highlight effect between version 24w38a and 24w39a (and later versions) without resource packs. Using a custom resource pack that modifies the highlight effect makes the display differences and layering order even more noticeable.

(Since 24w37a, the above phenomenon can be reproduced using a resource pack that modifies the highlight effect.)

What I expected to happen was...:
The highlight effect of empty slot outlines should be consistent and unified. There are two possible solutions: (1) All outlines should appear between slot_highlight_back.png and slot_highlight_front.png, which would make the outlines clearer when highlighted, although it would require modifying some empty slot outline sprites. (2) All outlines should appear below both slot_highlight_back.png and slot_highlight_front.png, which would prevent resource packs that modify the highlight effect from making empty slot outlines look like items.

What actually happened was...:
The highlight effect of empty slot outlines is inconsistent.