What’s the difference between feathering with “Shift + Mouse wheel scroll” and “Shape feathering radius” in the module parameter window? Is it fair to say that Feathering Radius does some edge detection while Shift + Scroll doesn’t?
Thanks!
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You are correct. Shift+mouse wheel is just your standard feather, while the feathering radius does edge detection (and does it really well).
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Thanks!
Yeah the day I discovered that functionality was the day my life changed 
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I always had the feeling it would behave like this, though I never put more thought into it.
Thank you for confirming
The way I learned to appreciate the difference in the controls was to display the mask and experiment with the sliders and options. It certainly behaved differently to what I expected. BTW, the reason I pick darktable over other great editing programs like RawTherapee and even Lightroom is the fantastic masking abilities in DT. I love that I can draw a really rough mask, apply a correction and then using the feathering and blurring radius sliders achieve an invisible transition. It reminds me of the days of dodging and burning in the darkroom with film.
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Also, the feathering options in the mask refinement section act on the overall mask (defined using one or more drawn shapes, parametric masks or a combination thereof), while the shape-specific feathering can be adjusted independently for each shape (and is not applicable to parametric masks).
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It’s a joy indeed to draw a mask, display it and then see what the refinement controls do 
BTW, the refinements work slightly differently with parametric masks compared to drawn masks. Experiment and learn. I love the masks in DT.