Thanks for answering:
In which document says that (about the definition of feather and blur)?
I know it’s not an official document, or it would change but according to wikipedia:“Feathering is a technique used in computer graphics software to smooth or blur the edges of a feature.”
en.wikipedia.org/wiki/Feathering
It seems to me, that feather it’s just a kind of blurring. And you say that featyher in Synfig is behaving like a blur. Also I see some difference between bluring in Inkscape, and Feathering In Synfig. In Synfig the border of a “feathered” geometry seems to “bright” the nearest regions.
Maybe We are a bit off-topic about UI Redesign?