GtkNotebook has a frame around the page that it shows, grouping that page’s widgets together. That makes sense when the notebook is part of a more complicated UI. But Glom uses a GtkNotebook for almost all of its main window, taking up all of the window’s width. The frame needs an inner border that wastes space and the nesting actually makes the UI look more complicated than it is.
That frame is not optional in GtkNotebook, so I had to create a custom Gtk::Notebook-like class to replace it in my code. The result is mostly better, though I don’t like those two toggle buttons much. Suggestions are welcome.
I will also move those top labels around, putting them in the same row as the List and Details toggle buttons, but this is a start.