Mask to Master script not working for me in Glyphs 3
you might want to report this in the scripts repository.
Try downloading the latest version from GitHub. We made changes two days ago that added support for Python 3. (https://github.com/justanotherfoundry/freemix-glyphsapp/pull/25)
It does not work 100% with components in the background. I was able to make simple components in the background work, but components in the background which in turn contain other components do not work as of yet.
You can find my preliminary work on the script here: https://github.com/florianpircher/freemix-glyphsapp/blob/patch-3/Mask%20to%20Master.py