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I think I found the problem.
Weirdly, when I tried doing what you say, and it wasn't working - the fWidth refused to stay set, so as soon as I deselected the object, the parameter instantly reset...

If you're sure you haven't done something wrong (you're not setting the fWidth parameter above 0.5 in an event or something?), maybe we need to check with Clickteam? I can't see why it would work on...

Now that is strange. Works fine for me on both DX9 and DX11, both with pre-multiplied alpha enabled and disabled.

I was thinking something like this:
https://1drv.ms/u/s!Atq7cUIJ7uextCG3lX4uE1F4ImmA?e=mYrpGE

So you still have control over the width of the wheel, but you don't have to change the edge softness...

I don't see how the alpha component can be outside the range 0-1. I reckon there's something else going on, but I don't know what... I'll have a look later, but I'm about to go out right now.

A...