Feat: Modal/Alert as focus sections — replace focusManager.clear() with section activation

ModalPresentationModifier and AlertPresentationModifier now register
dedicated focus sections (__modal__/__alert__) and activate them instead
of clearing the entire FocusManager. Modal buttons register in the modal
section. When the modal closes, the section is not re-registered and
endRenderPass() falls back to the previous active section automatically.

Sources/TUIkit/Modifiers/AlertPresentationModifier.swift

@@ -57,6 +57,9 @@ public struct AlertPresentationModifier<Content: View, Actions: View, Message: V
 // MARK: - Renderable
 
 extension AlertPresentationModifier: Renderable {
+    /// A stable section ID for alert focus sections.
+    private static var alertSectionID: String { "__alert__" }
+
     func renderToBuffer(context: RenderContext) -> FrameBuffer {
         // If not presented, just return base content
         guard isPresented.wrappedValue else {
@@ -82,6 +85,8 @@ extension AlertPresentationModifier: Renderable {
             actions: { actions }
         )
 
+        let focusManager = context.environment.focusManager
+
         // Render dimmed base with an isolated context.
         // The base content's buttons and key handlers register into a
         // throwaway FocusManager and KeyEventDispatcher so they don't
@@ -90,11 +95,19 @@ extension AlertPresentationModifier: Renderable {
         let isolatedContext = context.isolatedForBackground()
         let dimmedBuffer = TUIkit.renderToBuffer(dimmedBase, context: isolatedContext)
 
-        // Clear the real focus manager so the alert's buttons become
-        // the only registered focusables (auto-focus picks the first one).
-        context.environment.focusManager.clear()
+        // Register an alert focus section and activate it.
+        // The alert section becomes the active section, so Tab/arrows
+        // only navigate within the alert's focusable elements (buttons).
+        let sectionID = Self.alertSectionID
+        focusManager.registerSection(id: sectionID)
+        focusManager.activateSection(id: sectionID)
 
-        let alertBuffer = TUIkit.renderToBuffer(alert, context: context)
+        // Set the alert section in the context so child focusables
+        // (buttons in the alert) register in the alert section.
+        var alertContext = context
+        alertContext.activeFocusSectionID = sectionID
+
+        let alertBuffer = TUIkit.renderToBuffer(alert, context: alertContext)
 
         guard !dimmedBuffer.isEmpty else {
             return alertBuffer

Sources/TUIkit/Modifiers/ModalPresentationModifier.swift

@@ -42,12 +42,17 @@ public struct ModalPresentationModifier<Content: View, Modal: View>: View {
 // MARK: - Renderable
 
 extension ModalPresentationModifier: Renderable {
+    /// A stable section ID for modal focus sections.
+    private static var modalSectionID: String { "__modal__" }
+
     func renderToBuffer(context: RenderContext) -> FrameBuffer {
         // If not presented, just return base content
         guard isPresented.wrappedValue else {
             return TUIkit.renderToBuffer(content, context: context)
         }
 
+        let focusManager = context.environment.focusManager
+
         // Render dimmed base with an isolated context.
         // The base content's buttons and key handlers register into a
         // throwaway FocusManager and KeyEventDispatcher so they don't
@@ -56,11 +61,20 @@ extension ModalPresentationModifier: Renderable {
         let isolatedContext = context.isolatedForBackground()
         let dimmedBuffer = TUIkit.renderToBuffer(dimmedBase, context: isolatedContext)
 
-        // Clear the real focus manager so the modal's buttons become
-        // the only registered focusables (auto-focus picks the first one).
-        context.environment.focusManager.clear()
+        // Register a modal focus section and activate it.
+        // This replaces the previous focusManager.clear() approach.
+        // The modal section becomes the active section, so Tab/arrows
+        // only navigate within the modal's focusable elements.
+        let sectionID = Self.modalSectionID
+        focusManager.registerSection(id: sectionID)
+        focusManager.activateSection(id: sectionID)
 
-        let modalBuffer = TUIkit.renderToBuffer(modal, context: context)
+        // Set the modal section in the context so child focusables
+        // (buttons in the modal) register in the modal section.
+        var modalContext = context
+        modalContext.activeFocusSectionID = sectionID
+
+        let modalBuffer = TUIkit.renderToBuffer(modal, context: modalContext)
 
         guard !dimmedBuffer.isEmpty else {
             return modalBuffer