Discovered while testing MC-267860.

The bug

Flowing water and flowing lava use the center of the texture file for instances which are flowing directly north, south, west or east. It would therefore be expected that the side textures for flowing water and flowing lava would use this exact same region of the texture, as down is also an orthogonal direction. However, it instead uses the top left corner of the texture, which is unexpected. Only an 8×8 intersection of both regions is used in both cases, rather than the 16×16 center.

How to reproduce

1. Download and apply the attached resource pack
2. Place some water or (preferably due to the lack of a tint) lava on the ground and watch as it flows
3. Now place a contained column of lava or water (if using water, do not surround with blocks that cause it to use the "overlay" texture such as glass)
4. Compare the textures

Expected results

"center" would be visible as both the top texture of orthogonally flowing fluid and the side textures of downwards flowing fluid.

Actual resuts

"center" is correctly seen as the top texture of orthogonally flowing fluid. However, "corner" is seen on the sides of downwards fluid.

How to fix

Remap the side textures of fluids so they use the center of the texture file rather than the corner. Alternatively, rip out this system entirely and use the method detailed at the end of MC-267860.