Based on my testing, hud_batching works well with Forge 1.20.1 and mod version 1.5.2, but it results in extra black borders around the edges (including the vanilla inventory). This is likely due to transparent textures not blending properly with the background during rendering.

It doesn’t seem to be caused by this mod—could it be a vanilla feature? I hadn’t noticed it before.

The black border around the hotbar is a vanilla feature