Toplevel windows are leaked when closing a project
- Open the GTK Inspector
- Create a new project
- Add a GtkWindow
- Close the project (saving not required)
- Notice a new GtkWindow in the list of toplevels
- Show the window by changing it's visible property to true, you'll see it was the recently destroyed window
This affects both Builder and Glade, as it means each time we open a project we leave state behind.