After far too long I will inform anyone who is interested about the state of the new* futuristic flood fill functionality™.
To recap, the old new features were:
dilation/erosion - being able to grow or shrink the fill by setting an offset
feathering - being able to blur the fill
gap closing - being able to not seep through gaps smaller than an adjustable size (while still filling into corners)
Since then, I’ve also added the ability to use the erase and lock alpha modes when using the fill tool, which can be useful when it is easier to block-brush a region and remove contiguous sections on the outside of it. Both of these could previously be accomplished by working on a separate layer and then using blending modes and layer groups to mask, but this is definitely more ergonomic.
I have also finally got around to building and testing on a Windows VM and apart from needing to figure out (or more likely, just conditionally disable) multiprocessing for morph operations, it all seems to work fine after a couple of easily squashed bugs were dealt with.
There are a couple more things I want to do in this area (selectable constant source layer, better option adjustment), but for now I think the change set is large enough as it is.
* Guaranteed no more than 50 years old