Did you have any luck with this? I’m afraid I don’t have much more in the way of suggestions, aside from simply doing lots of print () statements or using a debugger, then comparing and contrasting the results against what you expect to see, until finally you have an “Aha!” moment.
As for the shader problem… Couldn’t you just go with an image after all? (When I gave it to you earlier, I didn’t know quite what you needed.) You now have some familiarity with fills, so you could just slap a bitmap paint on the circle and avoid that problem, I would assume. (The shader would make sense, on the other hand, if you later wanted to throw effects onto the quadrants, the center, etc.) I don’t have any Apple products myself, so I have zero idea what that issue might be, unless you just have an older build (Does it work without the isTimeDependent line? I believe one of the updates specifically addressed that.), in which case you could try one of the recent dailies.
Also, in criticism of my own shader… In this line:
if (len \> .9) return vec4(0.); // "outside", use clear color
I’m not really sure why I used .9 instead of 1. (That might give you a better fit, so you don’t need to scale your circle.) Actually, if it’s just being applied to a circle anyway, you could probably just strike that line.