I’m in my Heavy master trying to view the Regular master in the background, but the outlines are shown in the foreground. Is it mistake I’ve made in the settings? Could you help me please 
You are currently using the Select All 
tool which allows editing the current layer and any layer where the eye 
is open.
Hold on the Select All tool icon in the toolbar and choose the regular Select 
tool. There, layers with the eye open are shown in the background instead of being editable.
You can switch between the different select tools by pressing Shift-V. This is probably what you accidentally did to activate the Select All tool.
Oh my gosh 
Thank you so much, Florian!