cybereality Correct me if I'm wrong but I took a proper look at this and it seems to be about creating outlines only? It doesn't seem to address the issue of how to prevent overlap. My goal when looking at this is to have two different circles representing a radius and then as the radius expands the circles expand with it merging together if they overlap.
Now I think about it and this kind of surprised me but the most modern example I can find of this technique is Civilization VI, in this screenshot I searched up you can see though that the border even follows the terrain. See the border on the bottom right of the screenshot going over the hill.
I admit I have something of an obsession with this sort of effect, me and two other programmers managed to get it just about working in Unity and it even took some ridiculous shader trickery by making the engine think one type of maths was another but it is possible to do. I'm actually kind of hoping because of Godot's more open source nature there's a better way to implement it, although maybe that's wishful thinking.
Learning about merging shaders together this way is something I'd find particularly useful for the kind of effects I'm looking at because when you nail this effect for influence borders and the like it looks fantastic and clean. Even better if you could get a more 3D look from it.