While doing some edits in darktable 2.7 git d5716a, this suddenly appeared in front of me.
Exporting works fine, this seems to be affecting only the display frame.
Anyway, I leave it here for the records, since I really liked it.
I don’t know how to reproduce the steps that led to this, so I won’t fill a bug.
As for the original image, I was trying to do a monochrome with a dark sky and was getting issues regarding the sculpture, if I may name it so, probably because of masking x chromatic aberrations.
However, my edit amplified what I think is due to chromatic aberration (see the bright lines beneath the metallic structure). I tried tweaking lens correction before to no success. I think I will have to live with that, since my lens are really poor (EF kit lens from an old Canon SLR):
I see. You may refine the parametric masking in exposure 1 to include those areas. Mask refinement sliders (mask opacity, mask contrast, etc.) might be helpful here. Also, since chromatic aberration usually has high chroma values, you may also use saturation masking.