Hi Alex!
It seems that CombatEngineLayers.JUST_BELOW_WIDGETS layer is still below a bunch of really important world-space non-widget assets, like particles (from explosions, hits, and the like). It seems like the usual renderInWorldSpace() method renders above such things, but also above quite a few UI elements, while a layered renderer at JUST_BELOW_WIDGETS is indeed below those unwanted UI elements, but also below particles of any sort.
This is problematic for the distortion shader, since I have to choose between not distorting particles (really problematic for a lot of reasons) or distorting a significant subset of UI elements.
Great update, as a whole, though!