diff --git a/Sources/Example/ExampleApp.swift b/Sources/Example/ExampleApp.swift
index 80a65ef..04cef9c 100644
--- a/Sources/Example/ExampleApp.swift
+++ b/Sources/Example/ExampleApp.swift
@@ -1,14 +1,22 @@
 import Portico
 
-struct Counter: View {
-    @State private var count = 0
+private struct Item: Identifiable { let id: Int; let title: String }
+
+struct ListDemo: View {
+    @State private var items = [Item(id: 1, title: "One"),
+                                Item(id: 2, title: "Two"),
+                                Item(id: 3, title: "Three")]
+    @State private var next = 4
 
     var body: some View {
         VStack(spacing: 12) {
-            Label { "Count: \(count)" }
             HStack(spacing: 12) {
-                Button("-") { count -= 1 }
-                Button("+") { count += 1 }
+                Button("Add") { items.append(Item(id: next, title: "Item \(next)")); next += 1 }
+                Button("Remove first") { if !items.isEmpty { items.removeFirst() } }
+                Button("Reverse") { items.reverse() }
+            }
+            ForEach($items, id: \.id) { item in
+                Label(item.title)
             }
         }
     }
@@ -19,8 +27,8 @@ struct ExampleApp: App {
     var applicationId: String? { "dev.bscubed.PorticoExample" }
 
     var body: some Scene {
-        Window { _ in Counter() }
-            .title("Portico P1")
+        Window { _ in ListDemo() }
+            .title("Portico P4 — ForEach")
             .defaultSize(width: 1280, height: 800)
     }
 }
diff --git a/Sources/Portico/App/Window.swift b/Sources/Portico/App/Window.swift
index 11b4e7f..a16923e 100644
--- a/Sources/Portico/App/Window.swift
+++ b/Sources/Portico/App/Window.swift
@@ -83,6 +83,10 @@ public extension Window {
         let ctx = MountContext()
         let child = AnyView(content(window)).makeWidget(ctx)
         window.setContent(content: child)
+        _ = window.connectCloseRequest { [registry = ctx.registry] _ in
+            registry.teardown()
+            return false   // allow the default close to proceed
+        }
         window.present()
     }
 }
diff --git a/Sources/Portico/Containers/EitherView.swift b/Sources/Portico/Containers/EitherView.swift
new file mode 100644
index 0000000..2af5a8a
--- /dev/null
+++ b/Sources/Portico/Containers/EitherView.swift
@@ -0,0 +1,70 @@
+import Gtk
+
+/// A view that switches between two branches based on a `Binding<Bool>` condition.
+///
+/// Internally uses `Gtk.Stack` with named children `"true"` and `"false"`.
+/// Each branch's widgets are mounted at most once — the first time that
+/// branch becomes active. Subsequent condition changes only call
+/// `setVisibleChildName`.
+@MainActor public struct EitherView: View {
+    private let condition: Binding<Bool>
+    private let firstContent: [AnyView]
+    private let secondContent: [AnyView]
+
+    public var body: Never { fatalError() }
+
+    /// - Parameters:
+    ///   - condition: The binding that selects the visible branch.
+    ///   - first: The view for the `true` branch (mounted once, lazily).
+    ///   - second: The view for the `false` branch (mounted once, lazily).
+    public init(
+        _ condition: Binding<Bool>,
+        @ViewBuilder first: () -> [AnyView],
+        @ViewBuilder second: () -> [AnyView]
+    ) {
+        self.condition = condition
+        self.firstContent = first()
+        self.secondContent = second()
+    }
+}
+
+@_spi(Portico) extension EitherView: Mountable {
+    @_spi(Portico) public func mount(_ ctx: MountContext) -> Gtk.Widget {
+        let stack = Gtk.Stack()
+
+        // Branch widget caches — nil until the branch is first mounted.
+        var trueWidgets: [Gtk.Widget]? = nil
+        var falseWidgets: [Gtk.Widget]? = nil
+
+        // Mount the initially-active branch first so it's visible immediately.
+        if condition.wrappedValue {
+            trueWidgets = firstContent.map { $0.makeWidget(ctx) }
+            for w in trueWidgets! { _ = stack.addNamed(child: w, name: "true") }
+        } else {
+            falseWidgets = secondContent.map { $0.makeWidget(ctx) }
+            for w in falseWidgets! { _ = stack.addNamed(child: w, name: "false") }
+        }
+
+        // Track changes — re-evaluate condition, lazily mount the other
+        // branch on first flip.
+        let tracker = DependencyTracker { [stack] in
+            let isTrue = condition.wrappedValue
+            if isTrue {
+                if trueWidgets == nil {
+                    trueWidgets = firstContent.map { $0.makeWidget(ctx) }
+                    for w in trueWidgets! { _ = stack.addNamed(child: w, name: "true") }
+                }
+                stack.setVisibleChildName(name: "true")
+            } else {
+                if falseWidgets == nil {
+                    falseWidgets = secondContent.map { $0.makeWidget(ctx) }
+                    for w in falseWidgets! { _ = stack.addNamed(child: w, name: "false") }
+                }
+                stack.setVisibleChildName(name: "false")
+            }
+        }
+        tracker.run()
+        ctx.registry.add(tracker)
+        return stack
+    }
+}
diff --git a/Sources/Portico/Containers/ForEach.swift b/Sources/Portico/Containers/ForEach.swift
new file mode 100644
index 0000000..4c0dc3f
--- /dev/null
+++ b/Sources/Portico/Containers/ForEach.swift
@@ -0,0 +1,107 @@
+import Gtk
+
+/// A view that renders one row per element of a reactive array, keyed by a
+/// stable identifier, and applies a keyed diff on change — appending new rows,
+/// removing deleted rows, and reordering moved rows. Surviving rows are never
+/// rebuilt (widget identity and subscriptions are preserved).
+///
+/// Backed by a `Gtk.Box`. Reactivity requires a state-backed `Binding`
+/// (e.g. `$items`); a custom `Binding(get:set:)` does not track and will not
+/// update. Rows are built from an element value snapshot at insert time.
+@MainActor public struct ForEach<Element, ID: Hashable>: View {
+    private let data: Binding<[Element]>
+    private let id: KeyPath<Element, ID>
+    private let orientation: Gtk.Orientation
+    private let spacing: Int32
+    private let row: (Element) -> [AnyView]
+
+    public var body: Never { fatalError() }
+
+    /// - Parameters:
+    ///   - data: The reactive array driving the rows.
+    ///   - id: Key path to each element's stable identifier. IDs MUST be
+    ///     unique; on a duplicate the first occurrence wins and later ones are
+    ///     ignored for that diff.
+    ///   - orientation: Backing box orientation (default `.vertical`).
+    ///   - spacing: Spacing between rows in pixels (default 0).
+    ///   - row: Builds one row's views from an element value snapshot.
+    public init(
+        _ data: Binding<[Element]>,
+        id: KeyPath<Element, ID>,
+        orientation: Gtk.Orientation = .vertical,
+        spacing: Int32 = 0,
+        @ViewBuilder row: @escaping (Element) -> [AnyView]
+    ) {
+        self.data = data
+        self.id = id
+        self.orientation = orientation
+        self.spacing = spacing
+        self.row = row
+    }
+}
+
+@_spi(Portico) extension ForEach: Mountable {
+    @_spi(Portico) public func mount(_ ctx: MountContext) -> Gtk.Widget {
+        let box = Gtk.Box(orientation: orientation, spacing: spacing)
+
+        // Per-key on-screen state, persisted across diffs in the tracker closure.
+        var rows: [ID: (widgets: [Gtk.Widget], registry: NodeRegistry)] = [:]
+        var order: [ID] = []
+
+        func applyDiff() {
+            // Deduplicate by id (first occurrence wins), preserving input order.
+            var ordered: [(ID, Element)] = []
+            var seen = Set<ID>()
+            for e in data.wrappedValue {
+                let key = e[keyPath: id]
+                if seen.insert(key).inserted { ordered.append((key, e)) }
+            }
+            let newOrder = ordered.map { $0.0 }
+            let newSet = Set(newOrder)
+
+            // Removals: detach widgets, tear down the row's registry, drop it.
+            for key in order where !newSet.contains(key) {
+                if let r = rows[key] {
+                    for w in r.widgets { box.remove(child: w) }
+                    r.registry.teardown()
+                    ctx.registry.removeChild(r.registry)
+                    rows[key] = nil
+                }
+            }
+
+            // Insert new rows and enforce target order via running sibling.
+            var prev: Gtk.Widget? = nil
+            for (key, element) in ordered {
+                let entry: (widgets: [Gtk.Widget], registry: NodeRegistry)
+                if let existing = rows[key] {
+                    entry = existing
+                    var sib = prev
+                    for w in entry.widgets {
+                        box.reorderChildAfter(child: w, sibling: sib)
+                        sib = w
+                    }
+                } else {
+                    let child = ctx.makeChild()
+                    let widgets = row(element).map { $0.makeWidget(child) }
+                    entry = (widgets, child.registry)
+                    rows[key] = entry
+                    var sib = prev
+                    for w in entry.widgets {
+                        box.insertChildAfter(child: w, sibling: sib)
+                        sib = w
+                    }
+                }
+                prev = entry.widgets.last ?? prev
+            }
+            order = newOrder
+        }
+
+        // Initial mount + dependency registration happen inside the tracker
+        // run (same pattern as EitherView/OptionalView): applyDiff reads
+        // data.wrappedValue, registering this tracker with the backing box.
+        let tracker = DependencyTracker { applyDiff() }
+        tracker.run()
+        ctx.registry.add(tracker)
+        return box
+    }
+}
diff --git a/Sources/Portico/Containers/GtkRevealer+SingleChildContainer.swift b/Sources/Portico/Containers/GtkRevealer+SingleChildContainer.swift
new file mode 100644
index 0000000..9c3edc3
--- /dev/null
+++ b/Sources/Portico/Containers/GtkRevealer+SingleChildContainer.swift
@@ -0,0 +1,9 @@
+import Gtk
+
+@_spi(Portico) extension Gtk.Revealer: SingleChildContainer {
+    /// Delegates to `Gtk.Revealer.setChild(child:)` with the distinct SPI
+    /// name to support nullable child semantics (nil detaches).
+    @_spi(Portico) public func attachChild(_ child: Gtk.Widget?) {
+        setChild(child: child)
+    }
+}
diff --git a/Sources/Portico/Containers/OptionalView.swift b/Sources/Portico/Containers/OptionalView.swift
new file mode 100644
index 0000000..2a0ccc8
--- /dev/null
+++ b/Sources/Portico/Containers/OptionalView.swift
@@ -0,0 +1,42 @@
+import Gtk
+
+/// A view that shows or hides its child based on a `Binding<Bool>` condition.
+///
+/// Internally uses `Gtk.Revealer`. The child widget is mounted exactly once;
+/// subsequent condition changes only toggle `setRevealChild`.
+@MainActor public struct OptionalView: View {
+    private let condition: Binding<Bool>
+    private let content: [AnyView]
+
+    public var body: Never { fatalError() }
+
+    /// - Parameters:
+    ///   - condition: The binding that controls visibility.
+    ///   - content: The view to conditionally reveal (mounted once).
+    public init(
+        _ condition: Binding<Bool>,
+        @ViewBuilder content: () -> [AnyView]
+    ) {
+        self.condition = condition
+        self.content = content()
+    }
+}
+
+@_spi(Portico) extension OptionalView: Mountable {
+    @_spi(Portico) public func mount(_ ctx: MountContext) -> Gtk.Widget {
+        let revealer = Gtk.Revealer()
+        // Mount child once — build-once invariant.
+        for child in content {
+            revealer.attachChild(child.makeWidget(ctx))
+        }
+        // Apply initial state.
+        revealer.setRevealChild(revealChild: condition.wrappedValue)
+        // Track changes.
+        let tracker = DependencyTracker { [revealer] in
+            revealer.setRevealChild(revealChild: condition.wrappedValue)
+        }
+        tracker.run()
+        ctx.registry.add(tracker)
+        return revealer
+    }
+}
diff --git a/Sources/Portico/Core/GtkWidget+View.swift b/Sources/Portico/Core/GtkWidget+View.swift
new file mode 100644
index 0000000..2199767
--- /dev/null
+++ b/Sources/Portico/Core/GtkWidget+View.swift
@@ -0,0 +1,18 @@
+import Gtk
+
+/// Enables raw `Gtk.Widget` instances inside `@ViewBuilder` closures.
+///
+/// Any generated widget (`Gtk.Label`, `Gtk.Box`, …) can be used directly
+/// as a view without an explicit Portico wrapper:
+/// ```swift
+/// VStack {
+///     Gtk.Label(str: "Hello")   // ← raw widget drop-in
+/// }
+/// ```
+extension Gtk.Widget: View {
+    public var body: Never { fatalError() }
+}
+
+@_spi(Portico) extension Gtk.Widget: Mountable {
+    public func mount(_: MountContext) -> Gtk.Widget { self }
+}
diff --git a/Sources/Portico/Core/MountContext.swift b/Sources/Portico/Core/MountContext.swift
index 73b148c..e46b63c 100644
--- a/Sources/Portico/Core/MountContext.swift
+++ b/Sources/Portico/Core/MountContext.swift
@@ -1,9 +1,20 @@
-import Gtk
-
-/// Context threaded through the mount pipeline.
-///
-/// Empty in P0; reserved for the P4 signal/subscription registry needed
-/// for per-node teardown on unmount.
+/// Context threaded through the mount pipeline. Carries the ``NodeRegistry``
+/// that collects reactive resources for teardown.
 @_spi(Portico) @MainActor public final class MountContext {
-    @_spi(Portico) public init() {}
+    /// The registry for the current subtree; reactive sites register here.
+    @_spi(Portico) public let registry: NodeRegistry
+
+    /// Creates a root context with a fresh registry.
+    @_spi(Portico) public init() { self.registry = NodeRegistry() }
+
+    private init(registry: NodeRegistry) { self.registry = registry }
+
+    /// Returns a child context whose registry is an independently-torn-down
+    /// child of this context's registry. Used per `ForEach` row so removing a
+    /// row releases only that row's resources.
+    @_spi(Portico) public func makeChild() -> MountContext {
+        let child = NodeRegistry()
+        registry.addChild(child)
+        return MountContext(registry: child)
+    }
 }
diff --git a/Sources/Portico/Core/NodeRegistry.swift b/Sources/Portico/Core/NodeRegistry.swift
new file mode 100644
index 0000000..d180695
--- /dev/null
+++ b/Sources/Portico/Core/NodeRegistry.swift
@@ -0,0 +1,49 @@
+import Gtk
+
+/// Collects the reactive resources created while mounting one subtree so
+/// they can be released together when that subtree unmounts.
+///
+/// Holds subscription tokens, signal handles, and dependency trackers, plus
+/// child registries for independently-unmountable sub-subtrees (e.g. each
+/// `ForEach` row). `teardown()` cancels tokens, disconnects handles, tears
+/// down trackers, and recurses into children.
+@_spi(Portico) @MainActor public final class NodeRegistry {
+    private var tokens: [SubscriptionToken] = []
+    private var handles: [SignalHandle] = []
+    private var trackers: [DependencyTracker] = []
+    private var children: [NodeRegistry] = []
+
+    @_spi(Portico) public init() {}
+
+    /// Registers a subscription token to cancel on teardown.
+    @_spi(Portico) public func add(_ token: SubscriptionToken) { tokens.append(token) }
+
+    /// Registers a GObject signal handle to disconnect on teardown.
+    @_spi(Portico) public func add(_ handle: SignalHandle) { handles.append(handle) }
+
+    /// Registers a dependency tracker to detach from its state boxes on teardown.
+    @_spi(Portico) public func add(_ tracker: DependencyTracker) { trackers.append(tracker) }
+
+    /// Registers a child registry for an independently-unmountable sub-subtree.
+    @_spi(Portico) public func addChild(_ child: NodeRegistry) { children.append(child) }
+
+    /// Removes a child registry (called after the child has been torn down,
+    /// e.g. a removed `ForEach` row) so this registry no longer retains it.
+    @_spi(Portico) public func removeChild(_ child: NodeRegistry) {
+        children.removeAll { $0 === child }
+    }
+
+    /// Releases every registered resource and recurses into child registries.
+    /// Safe to call more than once (token cancel and handle disconnect are
+    /// idempotent).
+    @_spi(Portico) public func teardown() {
+        for t in tokens { t.cancel() }
+        tokens.removeAll()
+        for i in handles.indices { handles[i].disconnect() }
+        handles.removeAll()
+        for tr in trackers { tr.teardown() }
+        trackers.removeAll()
+        for c in children { c.teardown() }
+        children.removeAll()
+    }
+}
diff --git a/Sources/Portico/Core/View+Modifiers.swift b/Sources/Portico/Core/View+Modifiers.swift
new file mode 100644
index 0000000..f730c36
--- /dev/null
+++ b/Sources/Portico/Core/View+Modifiers.swift
@@ -0,0 +1,289 @@
+import Gtk
+@_spi(SGTKInternal) import GObject
+import CGObject
+
+// MARK: - C GValue helpers (private)
+
+/// Allocates a zero-initialized C `GValue` of the given type, calls `setup`,
+/// passes it to `body`, then unsets it.
+/// - Note: Inline helper to avoid `deinit` isolation issues under
+///   `StrictConcurrency=complete` with module-wide `@MainActor`.
+private func withGValue<R>(
+    gType: UInt,
+    setup: (UnsafeMutablePointer<GValue>) -> Void,
+    body: (UnsafeMutablePointer<GValue>) throws -> R
+) rethrows -> R {
+    var gvalue = GValue()
+    portico_g_value_init(&gvalue, gType)
+    defer { portico_g_value_unset(&gvalue) }
+    setup(&gvalue)
+    return try body(&gvalue)
+}
+
+@_silgen_name("g_value_init")
+private nonisolated func portico_g_value_init(_: UnsafeMutablePointer<GValue>, _: UInt)
+
+@_silgen_name("g_value_unset")
+private nonisolated func portico_g_value_unset(_: UnsafeMutablePointer<GValue>)
+
+@_silgen_name("g_value_set_string")
+private nonisolated func portico_g_value_set_string(_: UnsafeMutablePointer<GValue>, _: UnsafePointer<CChar>?)
+
+@_silgen_name("g_value_set_int")
+private nonisolated func portico_g_value_set_int(_: UnsafeMutablePointer<GValue>, _: Int32)
+
+@_silgen_name("g_value_set_boolean")
+private nonisolated func portico_g_value_set_boolean(_: UnsafeMutablePointer<GValue>, _: Int32)
+
+@_silgen_name("g_value_set_double")
+private nonisolated func portico_g_value_set_double(_: UnsafeMutablePointer<GValue>, _: Double)
+
+// MARK: - Preferred width
+
+extension View {
+    /// Sets the preferred width (static, applied once at mount).
+    public func preferredWidth(_ width: Int32) -> AnyView {
+        AnyView(makeWidget: { ctx in
+            let w = AnyView(self).makeWidget(ctx)
+            let (_, curH) = w.getSizeRequest()
+            w.setSizeRequest(width: width, height: curH)
+            return w
+        })
+    }
+
+    /// Sets the preferred width (reactive, updates on binding change).
+    public func preferredWidth(_ width: Binding<Int32>) -> AnyView {
+        AnyView(makeWidget: { ctx in
+            let w = AnyView(self).makeWidget(ctx)
+            let (_, curH) = w.getSizeRequest()
+            w.setSizeRequest(width: width.wrappedValue, height: curH)
+            ctx.registry.add(width.subscribe { [w] newW in
+                let (_, curH) = w.getSizeRequest()
+                w.setSizeRequest(width: newW, height: curH)
+            })
+            return w
+        })
+    }
+}
+
+// MARK: - Preferred height
+
+extension View {
+    /// Sets the preferred height (static, applied once at mount).
+    public func preferredHeight(_ height: Int32) -> AnyView {
+        AnyView(makeWidget: { ctx in
+            let w = AnyView(self).makeWidget(ctx)
+            let (curW, _) = w.getSizeRequest()
+            w.setSizeRequest(width: curW, height: height)
+            return w
+        })
+    }
+
+    /// Sets the preferred height (reactive, updates on binding change).
+    public func preferredHeight(_ height: Binding<Int32>) -> AnyView {
+        AnyView(makeWidget: { ctx in
+            let w = AnyView(self).makeWidget(ctx)
+            let (curW, _) = w.getSizeRequest()
+            w.setSizeRequest(width: curW, height: height.wrappedValue)
+            ctx.registry.add(height.subscribe { [w] newH in
+                let (curW, _) = w.getSizeRequest()
+                w.setSizeRequest(width: curW, height: newH)
+            })
+            return w
+        })
+    }
+}
+
+// MARK: - Horizontal expand
+
+extension View {
+    /// Sets whether the widget expands horizontally (static).
+    public func hexpand(_ expand: Bool) -> AnyView {
+        AnyView(makeWidget: { ctx in
+            let w = AnyView(self).makeWidget(ctx)
+            w.setHexpand(expand: expand)
+            return w
+        })
+    }
+
+    /// Sets whether the widget expands horizontally (reactive).
+    public func hexpand(_ expand: Binding<Bool>) -> AnyView {
+        AnyView(makeWidget: { ctx in
+            let w = AnyView(self).makeWidget(ctx)
+            w.setHexpand(expand: expand.wrappedValue)
+            ctx.registry.add(expand.subscribe { [w] v in
+                w.setHexpand(expand: v)
+            })
+            return w
+        })
+    }
+}
+
+// MARK: - Vertical expand
+
+extension View {
+    /// Sets whether the widget expands vertically (static).
+    public func vexpand(_ expand: Bool) -> AnyView {
+        AnyView(makeWidget: { ctx in
+            let w = AnyView(self).makeWidget(ctx)
+            w.setVexpand(expand: expand)
+            return w
+        })
+    }
+
+    /// Sets whether the widget expands vertically (reactive).
+    public func vexpand(_ expand: Binding<Bool>) -> AnyView {
+        AnyView(makeWidget: { ctx in
+            let w = AnyView(self).makeWidget(ctx)
+            w.setVexpand(expand: expand.wrappedValue)
+            ctx.registry.add(expand.subscribe { [w] v in
+                w.setVexpand(expand: v)
+            })
+            return w
+        })
+    }
+}
+
+// MARK: - Margin
+
+extension View {
+    /// Sets the margin on all four sides to the same value (static).
+    public func margin(_ m: Int32) -> AnyView {
+        AnyView(makeWidget: { ctx in
+            let w = AnyView(self).makeWidget(ctx)
+            w.setMarginStart(margin: m)
+            w.setMarginEnd(margin: m)
+            w.setMarginTop(margin: m)
+            w.setMarginBottom(margin: m)
+            return w
+        })
+    }
+
+    /// Sets the margin on all four sides to the same value (reactive).
+    public func margin(_ m: Binding<Int32>) -> AnyView {
+        AnyView(makeWidget: { ctx in
+            let w = AnyView(self).makeWidget(ctx)
+            let initial = m.wrappedValue
+            w.setMarginStart(margin: initial)
+            w.setMarginEnd(margin: initial)
+            w.setMarginTop(margin: initial)
+            w.setMarginBottom(margin: initial)
+            ctx.registry.add(m.subscribe { [w] v in
+                w.setMarginStart(margin: v)
+                w.setMarginEnd(margin: v)
+                w.setMarginTop(margin: v)
+                w.setMarginBottom(margin: v)
+            })
+            return w
+        })
+    }
+}
+
+// MARK: - CSS class
+
+extension View {
+    /// Adds a CSS class to the widget (static).
+    public func cssClass(_ name: String) -> AnyView {
+        AnyView(makeWidget: { ctx in
+            let w = AnyView(self).makeWidget(ctx)
+            w.addCssClass(cssClass: name)
+            return w
+        })
+    }
+
+    /// Adds a CSS class to the widget (reactive).
+    ///
+    /// When the binding changes, the old class is removed and the new one
+    /// is added.
+    public func cssClass(_ name: Binding<String>) -> AnyView {
+        AnyView(makeWidget: { ctx in
+            let w = AnyView(self).makeWidget(ctx)
+            let initial = name.wrappedValue
+            if !initial.isEmpty {
+                w.addCssClass(cssClass: initial)
+            }
+            var prev = initial
+            ctx.registry.add(name.subscribe { [w] newName in
+                guard newName != prev else { return }
+                w.removeCssClass(cssClass: prev)
+                if !newName.isEmpty {
+                    w.addCssClass(cssClass: newName)
+                }
+                prev = newName
+            })
+            return w
+        })
+    }
+}
+
+// MARK: - .bind escape hatch
+
+extension View {
+    /// Binds a string-typed value to a named GObject property.
+    public func bind(_ value: Binding<String>, to property: String) -> AnyView {
+        AnyView(makeWidget: { ctx in
+            let w = AnyView(self).makeWidget(ctx)
+            value.wrappedValue.withCString { cstr in
+                withGValue(gType: gTypeString, setup: { portico_g_value_set_string($0, cstr) }) { ptr in
+                    w.setProperty(propertyName: property, value: Value(takingOwnership: UnsafeMutableRawPointer(ptr)))
+                }
+            }
+            ctx.registry.add(value.subscribe { [w] newV in
+                newV.withCString { cstr in
+                    withGValue(gType: gTypeString, setup: { portico_g_value_set_string($0, cstr) }) { ptr in
+                        w.setProperty(propertyName: property, value: Value(takingOwnership: UnsafeMutableRawPointer(ptr)))
+                    }
+                }
+            })
+            return w
+        })
+    }
+
+    /// Binds an int32-typed value to a named GObject property.
+    public func bind(_ value: Binding<Int32>, to property: String) -> AnyView {
+        AnyView(makeWidget: { ctx in
+            let w = AnyView(self).makeWidget(ctx)
+            withGValue(gType: gTypeInt, setup: { portico_g_value_set_int($0, value.wrappedValue) }) { ptr in
+                w.setProperty(propertyName: property, value: Value(takingOwnership: UnsafeMutableRawPointer(ptr)))
+            }
+            ctx.registry.add(value.subscribe { [w] newV in
+                withGValue(gType: gTypeInt, setup: { portico_g_value_set_int($0, newV) }) { ptr in
+                    w.setProperty(propertyName: property, value: Value(takingOwnership: UnsafeMutableRawPointer(ptr)))
+                }
+            })
+            return w
+        })
+    }
+
+    /// Binds a boolean value to a named GObject property.
+    public func bind(_ value: Binding<Bool>, to property: String) -> AnyView {
+        AnyView(makeWidget: { ctx in
+            let w = AnyView(self).makeWidget(ctx)
+            withGValue(gType: gTypeBoolean, setup: { portico_g_value_set_boolean($0, value.wrappedValue ? 1 : 0) }) { ptr in
+                w.setProperty(propertyName: property, value: Value(takingOwnership: UnsafeMutableRawPointer(ptr)))
+            }
+            ctx.registry.add(value.subscribe { [w] newV in
+                withGValue(gType: gTypeBoolean, setup: { portico_g_value_set_boolean($0, newV ? 1 : 0) }) { ptr in
+                    w.setProperty(propertyName: property, value: Value(takingOwnership: UnsafeMutableRawPointer(ptr)))
+                }
+            })
+            return w
+        })
+    }
+
+    /// Binds a double value to a named GObject property.
+    public func bind(_ value: Binding<Double>, to property: String) -> AnyView {
+        AnyView(makeWidget: { ctx in
+            let w = AnyView(self).makeWidget(ctx)
+            withGValue(gType: gTypeDouble, setup: { portico_g_value_set_double($0, value.wrappedValue) }) { ptr in
+                w.setProperty(propertyName: property, value: Value(takingOwnership: UnsafeMutableRawPointer(ptr)))
+            }
+            ctx.registry.add(value.subscribe { [w] newV in
+                withGValue(gType: gTypeDouble, setup: { portico_g_value_set_double($0, newV) }) { ptr in
+                    w.setProperty(propertyName: property, value: Value(takingOwnership: UnsafeMutableRawPointer(ptr)))
+                }
+            })
+            return w
+        })
+    }
+}
diff --git a/Sources/Portico/Core/ViewBuilder.swift b/Sources/Portico/Core/ViewBuilder.swift
index ee79288..f93f17a 100644
--- a/Sources/Portico/Core/ViewBuilder.swift
+++ b/Sources/Portico/Core/ViewBuilder.swift
@@ -1,8 +1,9 @@
 /// Result builder for composing ``View`` hierarchies.
 ///
-/// Conditionals (`buildOptional`/`buildEither`) are passthrough in P0
-/// (they collapse to plain arrays) — correct for the static-only scope.
-/// P2 replaces them to emit `OptionalView`/`EitherView` nodes.
+/// `buildOptional`/`buildEither` are static-only passthroughs —
+/// they collapse to plain arrays. Dynamic conditionals use the
+/// explicit ``EitherView`` and ``OptionalView`` types which accept
+/// a `Binding<Bool>` and mount branches via `Gtk.Stack`/`Gtk.Revealer`.
 @resultBuilder public enum ViewBuilder {
     /// Wraps a single ``View`` in an ``AnyView``.
     public static func buildExpression<V: View>(_ v: V) -> [AnyView] { [AnyView(v)] }
diff --git a/Sources/Portico/State/DependencyTracker.swift b/Sources/Portico/State/DependencyTracker.swift
index a62d846..0500047 100644
--- a/Sources/Portico/State/DependencyTracker.swift
+++ b/Sources/Portico/State/DependencyTracker.swift
@@ -9,6 +9,7 @@
     @_spi(Portico) public static var current: DependencyTracker?
 
     private let body: () -> Void
+    private var deregistrations: [() -> Void] = []
 
     @_spi(Portico) public init(_ body: @escaping () -> Void) {
         self.body = body
@@ -25,4 +26,17 @@
 
     /// Re-evaluates the body; called by a state box when its value changes.
     @_spi(Portico) public func reevaluate() { run() }
+
+    /// Records how to detach this tracker from a state box it registered with.
+    /// Called by ``StateBox/get()`` on first registration.
+    @_spi(Portico) public func addDeregistration(_ deregister: @escaping () -> Void) {
+        deregistrations.append(deregister)
+    }
+
+    /// Detaches this tracker from every state box it depends on, so it stops
+    /// re-evaluating. Idempotent.
+    @_spi(Portico) public func teardown() {
+        for d in deregistrations { d() }
+        deregistrations.removeAll()
+    }
 }
diff --git a/Sources/Portico/State/StateBox.swift b/Sources/Portico/State/StateBox.swift
index 4676afe..9813219 100644
--- a/Sources/Portico/State/StateBox.swift
+++ b/Sources/Portico/State/StateBox.swift
@@ -18,7 +18,11 @@
     /// as a dependent, then returns the value.
     @_spi(Portico) public func get() -> Value {
         if let t = DependencyTracker.current {
-            dependents[ObjectIdentifier(t)] = t
+            let oid = ObjectIdentifier(t)
+            if dependents[oid] == nil {
+                dependents[oid] = t
+                t.addDeregistration { [weak self] in self?.dependents[oid] = nil }
+            }
         }
         return value
     }
diff --git a/Sources/Portico/Widgets/Button.swift b/Sources/Portico/Widgets/Button.swift
index 740cd1c..a210b55 100644
--- a/Sources/Portico/Widgets/Button.swift
+++ b/Sources/Portico/Widgets/Button.swift
@@ -3,9 +3,8 @@ import Gtk
 /// A button with a click action, backed by `Gtk.Button`.
 ///
 /// Supports text labels and icon-name variants. The `connectClicked`
-/// ``SignalHandle`` is intentionally discarded in P0 — the closure box
-/// is retained by GObject for the widget's lifetime. Per-node teardown
-/// is wired in P4.
+/// ``SignalHandle`` is registered with the mount context's ``NodeRegistry``
+/// for teardown on unmount.
 @MainActor public struct Button: View {
     private enum Kind {
         case label(String)
@@ -38,7 +37,7 @@ import Gtk
         case .icon(let s):  button = Gtk.Button(iconName: s)
         }
         let action = self.action
-        _ = button.connectClicked { _ in action() }
+        ctx.registry.add(button.connectClicked { _ in action() })
         return button
     }
 }
diff --git a/Sources/Portico/Widgets/Label.swift b/Sources/Portico/Widgets/Label.swift
index 2ef579d..7d87af0 100644
--- a/Sources/Portico/Widgets/Label.swift
+++ b/Sources/Portico/Widgets/Label.swift
@@ -45,14 +45,15 @@ import Gtk
             label.setText(str: s)
         case .binding(let b):
             label.setText(str: b.wrappedValue)
-            _ = b.subscribe { [label] v in
+            ctx.registry.add(b.subscribe { [label] v in
                 label.setText(str: v)
-            }
+            })
         case .dynamic(let make):
             let tracker = DependencyTracker { [label] in
                 label.setText(str: make())
             }
             tracker.run()
+            ctx.registry.add(tracker)
         }
         return label
     }
diff --git a/Tests/PorticoTests/ConditionalTests.swift b/Tests/PorticoTests/ConditionalTests.swift
new file mode 100644
index 0000000..d426261
--- /dev/null
+++ b/Tests/PorticoTests/ConditionalTests.swift
@@ -0,0 +1,168 @@
+import Testing
+@_spi(SGTKInternal) import Gtk
+@_spi(Portico) import Portico
+
+@MainActor @Suite(.serialized) struct ConditionalTests {
+
+    // MARK: - EitherView
+
+    @Test func eitherViewInitiallyTrueBranch() {
+        guard Gtk.initCheck() else { return }
+        ConditionProbe.reset()
+
+        let box = StateBox(true)
+        let binding = Binding(box)
+        let view = EitherView(binding,
+            first: { FirstProbeLabel() },
+            second: { SecondProbeLabel() }
+        )
+        let widget = AnyView(view).makeWidget(MountContext())
+        let stack = widget as! Gtk.Stack
+
+        // Initially, only the "true" branch is mounted and visible.
+        #expect(stack.getVisibleChildName() == "true")
+        #expect(ConditionProbe.firstCount == 1)
+        #expect(ConditionProbe.secondCount == 0)
+
+        // Toggle to false — lazy-mounts the "false" branch.
+        box.set(false)
+        #expect(stack.getVisibleChildName() == "false")
+        #expect(ConditionProbe.firstCount == 1)
+        #expect(ConditionProbe.secondCount == 1)
+
+        // Toggle back to true — no new mounts.
+        box.set(true)
+        #expect(stack.getVisibleChildName() == "true")
+        #expect(ConditionProbe.firstCount == 1)
+        #expect(ConditionProbe.secondCount == 1)
+    }
+
+    @Test func eitherViewInitiallyFalseBranch() {
+        guard Gtk.initCheck() else { return }
+        ConditionProbe.reset()
+
+        let box = StateBox(false)
+        let binding = Binding(box)
+        let view = EitherView(binding,
+            first: { FirstProbeLabel() },
+            second: { SecondProbeLabel() }
+        )
+        let widget = AnyView(view).makeWidget(MountContext())
+        let stack = widget as! Gtk.Stack
+
+        // Initially, only the "false" branch is mounted and visible.
+        #expect(stack.getVisibleChildName() == "false")
+        #expect(ConditionProbe.firstCount == 0)
+        #expect(ConditionProbe.secondCount == 1)
+
+        // Toggle to true — lazy-mounts the "true" branch.
+        box.set(true)
+        #expect(stack.getVisibleChildName() == "true")
+        #expect(ConditionProbe.firstCount == 1)
+        #expect(ConditionProbe.secondCount == 1)
+
+        // Toggle back to false — no new mounts.
+        box.set(false)
+        #expect(stack.getVisibleChildName() == "false")
+        #expect(ConditionProbe.firstCount == 1)
+        #expect(ConditionProbe.secondCount == 1)
+    }
+
+    @Test func eitherViewTracksConditionChanges() {
+        guard Gtk.initCheck() else { return }
+        ConditionProbe.reset()
+
+        let box = StateBox(true)
+        let binding = Binding(box)
+        let view = EitherView(binding,
+            first: { FirstProbeLabel() },
+            second: { SecondProbeLabel() }
+        )
+        let widget = AnyView(view).makeWidget(MountContext())
+        let stack = widget as! Gtk.Stack
+
+        // Three toggles: true→false→true→false
+        box.set(false)
+        box.set(true)
+        box.set(false)
+
+        #expect(stack.getVisibleChildName() == "false")
+        #expect(ConditionProbe.firstCount == 1)
+        #expect(ConditionProbe.secondCount == 1)
+    }
+
+    // MARK: - OptionalView
+
+    @Test func optionalViewMountsChildOnce() {
+        guard Gtk.initCheck() else { return }
+        ConditionProbe.reset()
+
+        let box = StateBox(true)
+        let binding = Binding(box)
+        let view = OptionalView(binding) { FirstProbeLabel() }
+        let widget = AnyView(view).makeWidget(MountContext())
+        let revealer = widget as! Gtk.Revealer
+
+        // Initially revealed, child mounted once.
+        #expect(revealer.getRevealChild() == true)
+        #expect(ConditionProbe.firstCount == 1)
+
+        // Toggle to false — child still mounted, just hidden.
+        box.set(false)
+        #expect(revealer.getRevealChild() == false)
+        #expect(ConditionProbe.firstCount == 1)
+
+        // Toggle back to true — still same child.
+        box.set(true)
+        #expect(revealer.getRevealChild() == true)
+        #expect(ConditionProbe.firstCount == 1)
+    }
+
+    @Test func optionalViewHidesWhenConditionFalse() {
+        guard Gtk.initCheck() else { return }
+        ConditionProbe.reset()
+
+        let box = StateBox(false)
+        let binding = Binding(box)
+        let view = OptionalView(binding) { FirstProbeLabel() }
+        let widget = AnyView(view).makeWidget(MountContext())
+        let revealer = widget as! Gtk.Revealer
+
+        // Initially hidden, but child WAS mounted (build-once).
+        #expect(revealer.getRevealChild() == false)
+        #expect(ConditionProbe.firstCount == 1)
+
+        // Toggle to true — child visible, no remount.
+        box.set(true)
+        #expect(revealer.getRevealChild() == true)
+        #expect(ConditionProbe.firstCount == 1)
+    }
+}
+
+// MARK: - Test helpers
+
+@MainActor fileprivate enum ConditionProbe {
+    static var firstCount = 0
+    static var secondCount = 0
+
+    static func reset() {
+        firstCount = 0
+        secondCount = 0
+    }
+}
+
+@MainActor fileprivate struct FirstProbeLabel: View, Mountable {
+    var body: Never { fatalError() }
+    func mount(_ ctx: MountContext) -> Gtk.Widget {
+        ConditionProbe.firstCount += 1
+        return Gtk.Label(str: "first")
+    }
+}
+
+@MainActor fileprivate struct SecondProbeLabel: View, Mountable {
+    var body: Never { fatalError() }
+    func mount(_ ctx: MountContext) -> Gtk.Widget {
+        ConditionProbe.secondCount += 1
+        return Gtk.Label(str: "second")
+    }
+}
diff --git a/Tests/PorticoTests/ForEachTests.swift b/Tests/PorticoTests/ForEachTests.swift
new file mode 100644
index 0000000..c855c0c
--- /dev/null
+++ b/Tests/PorticoTests/ForEachTests.swift
@@ -0,0 +1,179 @@
+import Testing
+@_spi(SGTKInternal) import Gtk
+@_spi(Portico) import Portico
+
+// MARK: - Probe types
+
+/// A label that records how many times it was mounted per key,
+/// for verifying ForEach rebuild-avoidance.
+private final class MountCounter {
+    var counts: [Int: Int] = [:]
+    func inc(_ key: Int) { counts[key, default: 0] += 1 }
+}
+
+private struct ProbeRow: View {
+    let key: Int
+    let label: String
+    let counter: MountCounter
+
+    var body: Never { fatalError() }
+
+    init(key: Int, label: String, counter: MountCounter) {
+        self.key = key; self.label = label; self.counter = counter
+    }
+}
+
+@_spi(Portico) extension ProbeRow: Mountable {
+    @_spi(Portico) public func mount(_ ctx: MountContext) -> Gtk.Widget {
+        counter.inc(key)
+        return Gtk.Label(str: label)
+    }
+}
+
+/// A shared mutable dictionary for tracking per-key subscription fires.
+private final class FireCounter {
+    var fires: [Int: Int] = [:]
+    func inc(_ key: Int) { fires[key, default: 0] += 1 }
+}
+
+/// A row that subscribes to a StateBox so we can verify the subscription
+/// is cancelled on row removal.
+private struct SubscribingRow: View {
+    let key: Int
+    let box: StateBox<Int>
+    let counter: FireCounter
+    var body: Never { fatalError() }
+
+    init(key: Int, box: StateBox<Int>, counter: FireCounter) {
+        self.key = key; self.box = box; self.counter = counter
+    }
+}
+
+@_spi(Portico) extension SubscribingRow: Mountable {
+    @_spi(Portico) public func mount(_ ctx: MountContext) -> Gtk.Widget {
+        ctx.registry.add(box.subscribe { [key, counter] _ in
+            counter.inc(key)
+        })
+        return Gtk.Label(str: "row \(key)")
+    }
+}
+
+// MARK: - Tests
+
+@MainActor @Suite(.serialized) struct ForEachTests {
+
+    // Helper: collect visible child labels from a Gtk.Box in order.
+    private func childLabels(_ box: Gtk.Box) -> [String] {
+        var result: [String] = []
+        var child = box.getFirstChild()
+        while let w = child {
+            // getFirstChild returns base Widget; reconstruct Label from pointer.
+            let lbl = Gtk.Label(retaining: w.pointer)
+            result.append(lbl.getText())
+            child = w.getNextSibling()
+        }
+        return result
+    }
+
+    // Helper: count the children of a Gtk.Box.
+    private func childCount(_ box: Gtk.Box) -> Int {
+        var n = 0
+        var child = box.getFirstChild()
+        while let w = child { n += 1; child = w.getNextSibling() }
+        return n
+    }
+
+    @Test func initialMountRendersRowsInOrder() {
+        guard Gtk.initCheck() else { return }
+        let items = StateBox([(1, "One"), (2, "Two"), (3, "Three")])
+        let binding = Binding(items)
+        let forEach = ForEach(binding, id: \.0) { (id: Int, title: String) in
+            Label(title)
+        }
+        let box = AnyView(forEach).makeWidget(MountContext()) as! Gtk.Box
+        #expect(childCount(box) == 3)
+        #expect(childLabels(box) == ["One", "Two", "Three"])
+    }
+
+    @Test func appendAddsRowWithoutRebuild() {
+        guard Gtk.initCheck() else { return }
+        let counter = MountCounter()
+        let items = StateBox([(1, "One"), (2, "Two"), (3, "Three")])
+        let binding = Binding(items)
+        let forEach = ForEach(binding, id: \.0) { (id: Int, title: String) in
+            ProbeRow(key: id, label: title, counter: counter)
+        }
+        let box = AnyView(forEach).makeWidget(MountContext()) as! Gtk.Box
+        #expect(childCount(box) == 3)
+        #expect(counter.counts[1] == 1)
+        #expect(counter.counts[2] == 1)
+        #expect(counter.counts[3] == 1)
+
+        var arr = items.get()
+        arr.append((4, "Four"))
+        items.set(arr)
+        #expect(childCount(box) == 4)
+        // Previously-mounted keys still counted once (never rebuilt).
+        #expect(counter.counts[1] == 1)
+        #expect(counter.counts[2] == 1)
+        #expect(counter.counts[3] == 1)
+        #expect(counter.counts[4] == 1)
+    }
+
+    @Test func removeDropsRowAndTearsDownSubscription() {
+        guard Gtk.initCheck() else { return }
+        let box = StateBox(0)
+        let fires = FireCounter()
+        let items = StateBox([(1, "One"), (2, "Two"), (3, "Three")])
+        let binding = Binding(items)
+
+        // Use actual Gtk.Labels with subscriptions that increment fires.
+        let forEach = ForEach(binding, id: \.0) { (id: Int, title: String) in
+            SubscribingRow(key: id, box: box, counter: fires)
+        }
+        let container = AnyView(forEach).makeWidget(MountContext()) as! Gtk.Box
+        #expect(childCount(container) == 3)
+
+        // Fire all: every row's subscription fires.
+        box.set(1)
+        #expect(fires.fires[1] == 1 && fires.fires[2] == 1 && fires.fires[3] == 1)
+
+        // Remove the middle element.
+        var arr = items.get()
+        arr.remove(at: 1) // remove key 2
+        items.set(arr)
+        #expect(childCount(container) == 2)
+        #expect(childLabels(container) == ["row 1", "row 3"])
+
+        // Fire again: removed row 2's subscription must NOT respond.
+        box.set(2)
+        #expect(fires.fires[1] == 2)
+        #expect(fires.fires[2] == 1) // unchanged — subscription was cancelled
+        #expect(fires.fires[3] == 2)
+    }
+
+    @Test func reorderPreservesWidgetsNoRebuild() {
+        guard Gtk.initCheck() else { return }
+        let counter = MountCounter()
+        let items = StateBox([(1, "One"), (2, "Two"), (3, "Three")])
+        let binding = Binding(items)
+        let forEach = ForEach(binding, id: \.0) { (id: Int, title: String) in
+            ProbeRow(key: id, label: title, counter: counter)
+        }
+        let box = AnyView(forEach).makeWidget(MountContext()) as! Gtk.Box
+        #expect(childLabels(box) == ["One", "Two", "Three"])
+        #expect(counter.counts[1] == 1)
+        #expect(counter.counts[2] == 1)
+        #expect(counter.counts[3] == 1)
+
+        // Reverse the array.
+        var arr = items.get()
+        arr.reverse()
+        items.set(arr)
+        #expect(childLabels(box) == ["Three", "Two", "One"])
+        // Mount counts unchanged — identity preserved.
+        #expect(counter.counts[1] == 1)
+        #expect(counter.counts[2] == 1)
+        #expect(counter.counts[3] == 1)
+    }
+}
diff --git a/Tests/PorticoTests/ModifierTests.swift b/Tests/PorticoTests/ModifierTests.swift
new file mode 100644
index 0000000..ea620c4
--- /dev/null
+++ b/Tests/PorticoTests/ModifierTests.swift
@@ -0,0 +1,248 @@
+import Testing
+@_spi(SGTKInternal) import Gtk
+@_spi(Portico) import Portico
+
+@MainActor @Suite(.serialized) struct ModifierTests {
+
+    // MARK: - Static modifiers
+
+    @Test func staticPreferredWidth() {
+        guard Gtk.initCheck() else { return }
+        let w = AnyView(Label("x").preferredWidth(42)).makeWidget(MountContext())
+        #expect(w.getSizeRequest().width == 42)
+    }
+
+    @Test func staticPreferredHeight() {
+        guard Gtk.initCheck() else { return }
+        let w = AnyView(Label("x").preferredHeight(99)).makeWidget(MountContext())
+        #expect(w.getSizeRequest().height == 99)
+    }
+
+    @Test func staticHexpand() {
+        guard Gtk.initCheck() else { return }
+        let w = AnyView(Label("x").hexpand(true)).makeWidget(MountContext())
+        #expect(w.getHexpand() == true)
+        let w2 = AnyView(Label("x").hexpand(false)).makeWidget(MountContext())
+        #expect(w2.getHexpand() == false)
+    }
+
+    @Test func staticVexpand() {
+        guard Gtk.initCheck() else { return }
+        let w = AnyView(Label("x").vexpand(true)).makeWidget(MountContext())
+        #expect(w.getVexpand() == true)
+    }
+
+    @Test func staticMargin() {
+        guard Gtk.initCheck() else { return }
+        let w = AnyView(Label("x").margin(7)).makeWidget(MountContext())
+        #expect(w.getMarginStart() == 7)
+        #expect(w.getMarginEnd() == 7)
+        #expect(w.getMarginTop() == 7)
+        #expect(w.getMarginBottom() == 7)
+    }
+
+    @Test func staticCssClass() {
+        guard Gtk.initCheck() else { return }
+        let w = AnyView(Label("x").cssClass("myclass")).makeWidget(MountContext())
+        #expect(w.hasCssClass(cssClass: "myclass"))
+    }
+
+    // MARK: - Reactive modifiers
+
+    @Test func reactivePreferredWidth() {
+        guard Gtk.initCheck() else { return }
+        let box = StateBox<Int32>(40)
+        let w = AnyView(Label("x").preferredWidth(Binding(box))).makeWidget(
+            MountContext())
+        defer { box.set(0) }
+
+        #expect(w.getSizeRequest().width == 40)
+        box.set(60)
+        #expect(w.getSizeRequest().width == 60)
+    }
+    @Test func reactiveCssClass() {
+        guard Gtk.initCheck() else { return }
+        let box = StateBox("a")
+        let w = AnyView(Label("x").cssClass(Binding(box))).makeWidget(
+            MountContext())
+        defer { box.set("") }
+
+        #expect(w.hasCssClass(cssClass: "a"))
+        box.set("b")
+        #expect(!w.hasCssClass(cssClass: "a"))
+        #expect(w.hasCssClass(cssClass: "b"))
+    }
+
+    @Test func reactivePreferredHeight() {
+        guard Gtk.initCheck() else { return }
+        let box = StateBox<Int32>(99)
+        let w = AnyView(Label("x").preferredHeight(Binding(box))).makeWidget(
+            MountContext())
+        _ = box
+
+        #expect(w.getSizeRequest().height == 99)
+        box.set(150)
+        #expect(w.getSizeRequest().height == 150)
+    }
+
+    @Test func reactiveHexpand() {
+        guard Gtk.initCheck() else { return }
+        let box = StateBox<Bool>(false)
+        let w = AnyView(Label("x").hexpand(Binding(box))).makeWidget(
+            MountContext())
+        _ = box
+
+        #expect(w.getHexpand() == false)
+        box.set(true)
+        #expect(w.getHexpand() == true)
+    }
+
+    @Test func reactiveVexpand() {
+        guard Gtk.initCheck() else { return }
+        let box = StateBox<Bool>(false)
+        let w = AnyView(Label("x").vexpand(Binding(box))).makeWidget(
+            MountContext())
+        _ = box
+
+        #expect(w.getVexpand() == false)
+        box.set(true)
+        #expect(w.getVexpand() == true)
+    }
+
+    @Test func reactiveMargin() {
+        guard Gtk.initCheck() else { return }
+        let box = StateBox<Int32>(5)
+        let w = AnyView(Label("x").margin(Binding(box))).makeWidget(
+            MountContext())
+        _ = box
+
+        #expect(w.getMarginStart() == 5)
+        #expect(w.getMarginEnd() == 5)
+        #expect(w.getMarginTop() == 5)
+        #expect(w.getMarginBottom() == 5)
+        box.set(12)
+        #expect(w.getMarginStart() == 12)
+        #expect(w.getMarginEnd() == 12)
+        #expect(w.getMarginTop() == 12)
+        #expect(w.getMarginBottom() == 12)
+    }
+
+    // MARK: - Axis preservation
+
+    @Test func axisPreservation() {
+        guard Gtk.initCheck() else { return }
+        let w = AnyView(
+            Label("x")
+                .preferredWidth(42)
+                .preferredHeight(99)
+        ).makeWidget(MountContext())
+        let sr = w.getSizeRequest()
+        #expect(sr.width == 42)
+        #expect(sr.height == 99)
+    }
+
+    @Test func reactiveWidthPreservesHeight() {
+        guard Gtk.initCheck() else { return }
+        let box = StateBox<Int32>(50)
+        let w = AnyView(
+            Label("x")
+                .preferredWidth(100)     // static height 0 (unset)
+                .preferredHeight(200)    // static width 100 preserved
+                .preferredWidth(Binding(box))  // reactive width
+        ).makeWidget(MountContext())
+        defer { box.set(0) }
+
+        // After static: width=100, height=200
+        // After reactive initial: width=50, height=200 (width overwritten, height preserved)
+        let sr = w.getSizeRequest()
+        #expect(sr.height == 200)
+
+        box.set(80)
+        let sr2 = w.getSizeRequest()
+        #expect(sr2.width == 80)
+        #expect(sr2.height == 200)
+    }
+
+    // MARK: - .bind modifier
+
+    @Test func bindBoolToVisible() {
+        guard Gtk.initCheck() else { return }
+        let box = StateBox<Bool>(true)
+        let w = AnyView(
+            Label("x").bind(Binding(box), to: "visible")
+        ).makeWidget(MountContext())
+        defer { box.set(false) }
+
+        #expect(w.getVisible() == true)
+        box.set(false)
+        #expect(w.getVisible() == false)
+    }
+
+    @Test func bindStringToTooltip() {
+        guard Gtk.initCheck() else { return }
+        let box = StateBox("hello")
+        let w = AnyView(
+            Label("x").bind(Binding(box), to: "tooltip-text")
+        ).makeWidget(MountContext())
+        defer { box.set("") }
+
+        #expect(w.getTooltipText() == "hello")
+        box.set("world")
+        #expect(w.getTooltipText() == "world")
+    }
+
+    @Test func bindIntToWidthRequest() {
+        guard Gtk.initCheck() else { return }
+        let box = StateBox<Int32>(50)
+        let w = AnyView(
+            Label("x").bind(Binding(box), to: "width-request")
+        ).makeWidget(MountContext())
+        _ = box
+
+        #expect(w.getSizeRequest().width == 50)
+        box.set(120)
+        #expect(w.getSizeRequest().width == 120)
+    }
+
+    @Test func bindDoubleToOpacity() {
+        guard Gtk.initCheck() else { return }
+        let box = StateBox<Double>(1.0)
+        let w = AnyView(
+            Label("x").bind(Binding(box), to: "opacity")
+        ).makeWidget(MountContext())
+        _ = box
+
+        #expect(w.getOpacity() == 1.0)
+        box.set(0.0)
+        #expect(w.getOpacity() == 0.0)
+    }
+
+    // MARK: - Raw widget drop-in
+
+    @Test func rawWidgetInVStack() {
+        guard Gtk.initCheck() else { return }
+        let raw = Gtk.Label(str: "raw")
+        let container = AnyView(VStack(spacing: 0) { raw }).makeWidget(
+            MountContext())
+
+        // VStack is backed by Gtk.Box — verify the raw label is a child
+        // GTK wrappers create new Swift objects for the same GObject, so
+        // compare the underlying object pointers instead of using ===.
+        var found = false
+        var cur = container.getFirstChild()
+        while let c = cur {
+            if c.pointer == raw.pointer { found = true; break }
+            cur = c.getNextSibling()
+        }
+        #expect(found)
+    }
+
+    @Test func rawWidgetModifierChain() {
+        guard Gtk.initCheck() else { return }
+        let raw = Gtk.Label(str: "chained")
+        let w = AnyView(raw.preferredWidth(100).hexpand(true)).makeWidget(
+            MountContext())
+        #expect(w.getSizeRequest().width == 100)
+        #expect(w.getHexpand() == true)
+    }
+}
diff --git a/Tests/PorticoTests/TeardownTests.swift b/Tests/PorticoTests/TeardownTests.swift
new file mode 100644
index 0000000..43af95c
--- /dev/null
+++ b/Tests/PorticoTests/TeardownTests.swift
@@ -0,0 +1,57 @@
+import Testing
+@_spi(SGTKInternal) import Gtk
+@_spi(Portico) import Portico
+
+@MainActor @Suite struct TeardownTests {
+
+    // MARK: - No GTK needed
+
+    @Test func nodeRegistryCancelsTokens() {
+        let box = StateBox(0)
+        var count = 0
+        let registry = NodeRegistry()
+        registry.add(box.subscribe { _ in count += 1 })
+        box.set(1)
+        #expect(count == 1)
+        registry.teardown()
+        box.set(2)
+        #expect(count == 1)
+    }
+
+    @Test func trackerTeardownStopsReevaluation() {
+        let box = StateBox(0)
+        var evaluateCount = 0
+        let tracker = DependencyTracker { _ = box.get(); evaluateCount += 1 }
+        tracker.run()
+        #expect(evaluateCount == 1)
+        box.set(1)
+        #expect(evaluateCount == 2)
+        tracker.teardown()
+        box.set(2)
+        #expect(evaluateCount == 2)
+    }
+
+    @Test func nodeRegistryTeardownIsIdempotent() {
+        let box = StateBox(0)
+        var count = 0
+        let registry = NodeRegistry()
+        registry.add(box.subscribe { _ in count += 1 })
+        box.set(1)
+        #expect(count == 1)
+        registry.teardown()
+        registry.teardown() // must not crash
+        box.set(2)
+        #expect(count == 1)
+    }
+
+    // MARK: - GTK-dependent
+
+    @Test(.serialized) func nodeRegistryDisconnectsSignalHandle() {
+        guard Gtk.initCheck() else { return }
+        let button = Gtk.Button(label: "test")
+        let ctx = MountContext()
+        ctx.registry.add(button.connectClicked { _ in })
+        // Teardown must not crash with a real SignalHandle.
+        ctx.registry.teardown()
+    }
+}