The concept of a "maskable adjustment layer" came up in conversation - where you can selectively mask an NDE filter on the layers below. I realized that after @npub1wvm3...5rdg 's work on making passthrough layer groups cover the full group render, we basically have this already. With a minor code tweak I'll be submitted soon, you can recreate maskable adjustment layers right in GIMP!

A WIP UI for non-destructive filters applied to layer masks. This is the tricky part - the actual implementation code was pretty simple. :) I'm not sure if the switcher should always appear, even if the mask (or layer) has no filters, or only if both the layer and mask have active filters. Feedback appreciated!