Fix child-adder code generation

Tests/PorticoTests/GeneratedWidgetTests.swift

@@ -1,6 +1,6 @@
 import Testing
 import Adw
-import Gtk
+@_spi(SGTKInternal) import Gtk
 @_spi(Portico) import Portico
 
 @MainActor @Suite(.serialized) struct GeneratedWidgetTests {
@@ -183,4 +183,71 @@ import Gtk
         box.set(100)
         #expect(avatar.getSize() == 48, "size should not change after teardown")
     }
+    // MARK: - Child-adder generated APIs
+
+    /// Two rows plus a header suffix, proving children land as rows and the slot still binds.
+    @Test func preferencesGroupChildrenAndHeaderSuffix() {
+        guard Gtk.initCheck() else { return }
+        let w = AnyView(PreferencesGroup {
+            ActionRow()
+            ActionRow()
+        } headerSuffix: {
+            Label(str: "suffix")
+        }).makeWidget(MountContext())
+        guard let group = w as? Adw.PreferencesGroup else {
+            Issue.record("Expected Adw.PreferencesGroup, got \(type(of: w))")
+            return
+        }
+        #expect(group.getRow(index: 0) != nil)
+        #expect(group.getRow(index: 1) != nil)
+        #expect(group.getRow(index: 2) == nil)
+        #expect(group.getHeaderSuffix() != nil)
+    }
+
+    /// The generated `children:` builder adds children in closure order.
+    @Test func stackChildrenInitAddsInOrder() {
+        guard Gtk.initCheck() else { return }
+        let w = AnyView(Stack {
+            Label(str: "first")
+            Label(str: "second")
+        }).makeWidget(MountContext())
+        guard let stack = w as? Gtk.Stack else {
+            Issue.record("Expected Gtk.Stack, got \(type(of: w))")
+            return
+        }
+        // getFirstChild returns a base Widget; reconstruct Label from the pointer.
+        guard let a = stack.getFirstChild(), let b = a.getNextSibling() else {
+            Issue.record("Expected two children")
+            return
+        }
+        #expect(Gtk.Label(retaining: a.pointer).getText() == "first")
+        #expect(Gtk.Label(retaining: b.pointer).getText() == "second")
+        #expect(b.getNextSibling() == nil)
+    }
+
+    /// `Adw.Squeezer` has no `Widget` slot, so its whole content API is generated from
+    /// `add(child:)`; this also exercises the static `Gtk.Widget` modifier overload.
+    @Test func squeezerStaticAddModifierAddsChild() {
+        guard Gtk.initCheck() else { return }
+        let child = Gtk.Button(label: "a")
+        let w = AnyView(Squeezer().add(child)).makeWidget(MountContext())
+        guard let squeezer = w as? Adw.Squeezer else {
+            Issue.record("Expected Adw.Squeezer, got \(type(of: w))")
+            return
+        }
+        #expect(squeezer.getVisibleChild()?.pointer == child.pointer)
+    }
+
+    /// `Adw.ViewStack.visibleChild` is deliberately not a slot; the children builder is
+    /// what makes a page visible.
+    @Test func viewStackChildrenInitMakesChildVisible() {
+        guard Gtk.initCheck() else { return }
+        let w = AnyView(ViewStack { Label(str: "x") }).makeWidget(MountContext())
+        guard let stack = w as? Adw.ViewStack else {
+            Issue.record("Expected Adw.ViewStack, got \(type(of: w))")
+            return
+        }
+        #expect(stack.getVisibleChild() != nil)
+    }
+
 }