Masking with Channel Mixer
darktable 3.1.0+2385~gbea8af41b ... Manjaro/Arch/XFCE
Applying a mask before using any of the presets in channel-mixer ... the mask is entirely removed and the preset is applied over the full image. Problem
is seen with line, brush and shape masks.
The problem is resolved by first applying the preset over the entire image and then adding a mask.