Foundation implementation

Tests/PorticoTests/MountTests.swift

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+import Testing
+import Gtk
+@_spi(Portico) import Portico
+
+@MainActor @Suite(.serialized) struct MountTests {
+    private func childCount(_ w: Gtk.Widget) -> Int {
+        var n = 0
+        var cur = w.getFirstChild()
+        while let c = cur { n += 1; cur = c.getNextSibling() }
+        return n
+    }
+
+    @Test func vstackAppendsChildrenInOrder() {
+        guard Gtk.initCheck() else { return }
+        let widget = AnyView(VStack(spacing: 0) {
+            Label("a")
+            Button("b") {}
+        }).makeWidget(MountContext())
+        #expect(childCount(widget) == 2)
+    }
+
+    @Test func hstackMountsLabelAndText() {
+        guard Gtk.initCheck() else { return }
+        let widget = AnyView(HStack(spacing: 0) {
+            Label("a")
+            Text("b")
+        }).makeWidget(MountContext())
+        #expect(childCount(widget) == 2)
+    }
+
+    @Test func buttonStoresAction() {
+        // Validates Portico-level contract: Button stores its action
+        // closure, and it is callable synchronously. The connectClicked
+        // signal wiring cannot be tested synchronously in P0 — GTK's
+        // activate→clicked chain requires a realized widget and main-loop
+        // iteration (~250ms timer), out of scope for unit tests.
+        var fired = false
+        let button = Button("x") { fired = true }
+        button.action()
+        #expect(fired)
+    }
+}

Tests/PorticoTests/ViewBuilderTests.swift

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+import Testing
+@_spi(Portico) import Portico
+
+@Suite struct ViewBuilderTests {
+    @ViewBuilder private func mixed(_ flag: Bool) -> [AnyView] {
+        Label("a")
+        if flag { Label("b") }
+    }
+
+    @ViewBuilder private func either(_ flag: Bool) -> [AnyView] {
+        if flag { Label("a") } else { Label("b"); Label("c") }
+    }
+
+    @ViewBuilder private func loop() -> [AnyView] {
+        for _ in 0..<3 { Label("x") }
+    }
+
+    @Test func mixedPlainAndConditional() {
+        #expect(mixed(true).count == 2)
+        #expect(mixed(false).count == 1)
+    }
+
+    @Test func eitherBranches() {
+        #expect(either(true).count == 1)
+        #expect(either(false).count == 2)
+    }
+
+    @Test func forLoopFlattens() {
+        #expect(loop().count == 3)
+    }
+}

Tests/PorticoTests/WindowConfigTests.swift

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+import Testing
+@_spi(Portico) import Portico
+
+@Suite struct WindowConfigTests {
+    @Test func modifiersAccumulate() {
+        let w = Window { _ in Label("x") }
+            .title("T")
+            .defaultSize(width: 100, height: 200)
+            .resizable(false)
+            .fullscreen()
+        #expect(w.config.title == "T")
+        #expect(w.config.defaultSize?.width == 100)
+        #expect(w.config.defaultSize?.height == 200)
+        #expect(w.config.resizable == false)
+        #expect(w.config.fullscreen == true)
+    }
+
+    @Test func defaultConfigIsEmpty() {
+        let w = Window { _ in Label("x") }
+        #expect(w.config.title == nil)
+        #expect(w.config.defaultSize == nil)
+        #expect(w.config.resizable == nil)
+        #expect(w.config.fullscreen == false)
+    }
+}