Discovered while testing the fixes to MC-4533 and MC-70848.

The bug

Certain configurations of blocks can still make water appear to be flowing slightly upwards. This is generally due to the presence of orthogonally-adjacent blocks, as removing said blocks often results in this upwards-flowing shape changing to something more believable.

How to reproduce

1. Recreate the following structure:
2. Note how the water appears to flow upwards at the corner when two blocks are placed at it as such, yet does not appear to flow upwards if absent.

Expected results

Fluids wouldn't flow in illogical-looking ways.

Actual results

They do in this situation.

How to fix

I'm not sure if making fluids not change shape in response to solid blocks would help anything, and instead only make them change in response to other fluids of the same type. This, as well as defining more fluid shapes instead of having them be created on the fly, may resolve this issue somewhat.

Converting fluids over to using models like blocks do would be a great solution, allowing for resource pack customization as well as allowing the then-useless fluid height rendering code to be removed. Doing triangles to emulate how fluids currently look may prove difficult, however, so it may require changing fluids to appear more blocky and cuboidal, which would bring them more in line with other blocks but would likely end up as a controversial design change.