Hopefully I am not asking something stupid here, but I could not find this documented anywhere. I am trying to make a rectangular solder mask opening with rounded edges. There will be no component there, it is just an opening to allow some light to shine through the -otherwise black- soldermask.
I proceeded to create a fill plane on the F.Mask layer and set the outline display to “line” and the minimum width to 2mm to create 1mm radiuses. This kind of worked, because the interface is showing what I expected to see: