This is Part 3 of a series that takes a GNOME application from an empty directory to acceptance into GNOME Circle. Part 2 covered GObject's type system — properties, signals, and the inner/outer type pattern. Now we'll use everything we learned to build a real application. From theory to a running window In Part 2 we built a GObject subclass by hand — a Feed model with properties and signals, no GTK in sight. That was deliberate. Understanding GObject's inner/outer type split, the ObjectSubclass trait, and the mod imp pattern is the foundation that everything else rests on. Now we're going to see those same patterns in context. By the end of this post, you'll have a running GTK4 + libadwaita application with a header bar, a menu, keyboard shortcuts, and a properly structured project that's ready for Flatpak packaging. And you won't have written most of it by hand — GNOME Builder will generate it for you. Our job is to understand what it generated and why.…