Tested with and without Optifine and found this...

Seems like the liquid knows that there's a barrier to stop it from spreading everywhere, but still acts like there isn't (all the of the lava blocks are source blocks; I have used WorldEdit to make this). It's left that obvious gap, which does, in a modded parkour map, look quite ugly.

I hope that you can fix this quick. :)

It's a known bug with fancy fluids.. it's a very buggy implementation since it's hard to make it just right. Next version will try to improve it.

I model it around the BlockLiquid.class, and unfortunately it's hard-coded to always render the top side. Like I said, no perfect way to handle this.

Another bug involving pools of transparent liquids that are more than one block deep.

I'm using setting 3 for routable fluids (works fine for me - no lag spikes AND it fixes the bug previously mentioned (see screenshot above the one in this post)!).

As far as I can see, there must be something that can detect whether a block that is using routable fluids is directly above it (one block only) which needs to be implemented.

I know that, but it could also use and extend a rendering class (no actual changes to maintain OF and ShadersModCore support).

Or it could just be implemented as connected textures.