I found several threads on this topic from earlier in the year (March/April). In those threads, they indicating that feathering was not working properly but was now fixed.
I'm on v8.96 (tried installed 8.97 beta, but still says 8.96 in the app), and I'm still seeing the gray outline that gets larger as you increase feathering. Specifically, I have a polygon mask outline around a person and then blur the background. With the normal blur, I get a very noticeable transition from my polygon mask to the background. Radial blur helps with the transition, but the background is of course more warped on the blur (as expected for a radial blur).
Assuming the blur has not come back, should feathering be blending on the transition on the mask outline, or is it supposed to show the gray outline increasing with higher levels of feathering?
P.S. I won't be able to upload the video I was editing (helping a friend edit a family video with their child). If needed, I can try to recreate on a generic video.
