Hi!

I have a fluid-container, and I have a rigid-body object interacting with the contained fluid. Creating a splash.
Really original

My questions is, how do I get the splash-particles to NOT collide with container walls, but continue their “travel” out of the container instead of crashing with the container wall ?

I have Waterline turned on, but it does not seem to do anyting

Flip can only exist in the container. So you need to make another mesh based box as a collision container for what you have shown in the image. Then extend the FLIP fluid area beyond that. The water line will only get you the option of having flip below the line collide with the wall and everything above escape and disappear.