This is Part 4 of a series taking a GNOME app from an empty directory to GNOME Circle. Part 3 walked through every file Builder generated for our gazette project. Now we're going to start changing things. If you're new to this stack and wondering why GTK and libadwaita are separate libraries, why GObject's type system feels like 1990s C, or why Flatpak ships its own runtime alongside your app, there's a short companion piece on the history of the stack . Skim it for context or skip it for code. The XML problem At the end of Part 3, we had a working app. A header bar, a hamburger menu, a "Hello, World!" label, and a window.ui file that looked like this: <?xml version="1.0" encoding="UTF-8"?> <interface> <requires lib= "gtk" version= "4.0" /> <requires lib= "Adw" version= "1.0" /> <template class= "GazetteWindow" parent= "AdwApplicationWindow" > <property name= "title" translatable= "yes" > Gazette </property> <property name= "default-width" > 800…