class CustomPaintable implements Gdk.Paintable {
  // Implement all methods manually
  get_current_image(): Gdk.Paintable { ... }
  get_flags(): Gdk.PaintableFlags { ... }
  get_intrinsic_width(): number { ... }
  // ... and many more
}

class CustomPaintable implements Gdk.Paintable.Interface {
  // Declare for TypeScript compatibility
  declare get_current_image: Gdk.Paintable["get_current_image"];
  declare get_flags: Gdk.Paintable["get_flags"];
  
  // Only implement virtual methods
  vfunc_get_current_image(): Gdk.Paintable { ... }
  vfunc_get_flags(): Gdk.PaintableFlags { ... }
}