Just curious if that’s possible with the bottom preview bar (or makes any sense as a feature request for future)?
Basically, a scenario would be having an extreme Thin and Black master, but wanting to see not just the Regular interpolation in the preview bar, but also the Bold interpolation at the same time… so as to compare them visually together and how they relate.
I typically do this by having to export the full family and then compare in Illustrator or InDesign. Works, but could be nice to see and tweak the weight values in Glyphs app first.