This topic is a suggestion, and is also intended to open a discussion if desired by others.
I always have the letter shape visible next to the letter I am editing; that way I can see the mass and correct mistakes more easily (img). Is it possible to create a mode ‘view > fill counters’, just like we have the menu ‘view > fill preview’? That way it becomes more easily to see what is worked on, without having to look to a different place. In this mode, the counters could disappear and the nodes should be less visible (smaller/translucent/…)
It is closely related to the new plugin for Glyphs presented on ATypI 2017 and to the Lecture of Tom Grace, in which he explained the trouble of working with outlines. Things like the preview panel are not working because that is to far away from the place we’re working…
I’m still not sure what is being asked for because counters, being outside a path, cannot be filled. They can be reversed/inverted, but I don’t believe that’s what is being asked for.
Something akin to the image can be accomplished simply through use of the Text Tool, putting any character one wants on either or both sides of the working glyph.