The bug

The textures which water and lava use for "still" blocks are defined through the "particle" texture in the block model files. This is unintuitive and disallows customizing the particle textures of these blocks separately from their own textures.

How to reproduce

1. Apply the attached resource pack: test-water-lava-21w43a.zip
2. Find any existing water or lava
3. Note that still water looks like blue stone, and lava looks like oak planks

Expected results

Changing the particle texture in water/lava models would not change how their still textures look. Rather, this would be specified through a different entry in the model file, possibly also with a way to change the flowing texture.

Actual results

Changing the particle texture does in fact change the texture associated with still fluids.

How to fix

This could be fixable by making fluids customizable via models like blocks and items are, however this would likely require many other changes.

Perhaps said model might not allow for the customization of the fluid's geometry, but rather just the textures used for each face - top, north, south, east, west and bottom, as well as flowing top, flowing bottom, and glass-occluded north, south, east and west.