In clear physics mode, one-way walls have arrows to show their existence, but it would be better still if their terrain pixels were also recoloured to show their precise boundaries. That would, for example, allow the player to know whether it's possible to bash in between a OWW and a deadly object without having to experiement.
This is a good idea, but I'd also argue it's essentially a subset of this proposal: https://www.lemmingsforums.net/index.php?topic=4387.0
Nonetheless, I'll leave this topic open because one-way-arrows are a specific case where this would offer far more benefit, than it would for another average object type.
Should the different directions also have different colors? Or perhaps even just different shades of the same color?
(EDIT: V12.11 will have a solution for this. Specifically, when you mouse-over a one way wall, *all* one-way terrain of that direction in the level gets highlit.)