Consider the following gradient:
Now I create a solid color layer and copy-paste the gradient as its layer mask.
Where the mask is white I got 100% opacity and where the mask is black I got 0% opacity, with a fading in between.
The result looks like this (visibility for the gradient layer is off):
At this point I duplicate the solid color layer and invert the mask of the new layer (
Colors → Invert in GIMP).
What I expect is that the opacity of the new layer should be complementary to the first one, so having the two layers stacked one on top of the other should result in a solid color.
This is what I get instead:
Why is this happening and how to get what I want instead?