I have a TIFF of the night sky over a mountain (6720x4480), which I stacked using Starry Landscape Stacker. I also saved the stacked sky image "mask" with the mountain cut out, but not PERFECTLY (there's a slight gap between the mountain and the mask that's a little wider in some places than others).
I'd like to adjust the sky and mountain separately, and then put them back together smoothly. I've tried using the mask image to create a selection, shrinking and feathering to try to make the edge smooth, and inverting to get the other part, but no matter what I do once I adjust and reassemble there's either a prominent light or dark line along the border.
I'm completely new to any sort of photo editing software like this, and even the "for beginners" answers and tutorials leave me completely in the dark about what I'm doing wrong. I'm trying to use GIMP (2.10) on a MacBook, partly because I don't want to sink money into software that it's starting to feel like I'm too stupid to use right.