Use this skill when building Avalonia or WPF desktop applications, creating cross-platform desktop UI, or working with .axaml files. Covers Avalonia 11.x framework setup, layout system (Grid, StackPanel, DockPanel, WrapPanel, Canvas), core controls (Button, TextBox, ComboBox, DataGrid, TreeView, TreeDataGrid, ItemsRepeater, ToggleSwitch, SplitButton), CSS-like styling and theming (Fluent theme, Simple theme, light/dark mode, ControlThemes, style selectors, pseudo-classes), data binding (compiled bindings, x:DataType, element/self/parent binding, converters), MVVM architecture (CommunityToolkit.Mvvm, ReactiveUI, ViewLocator, DI setup, navigation patterns, dialog services), custom controls (UserControl, TemplatedControl, StyledProperty, DirectProperty, AttachedProperty), events and input handling (routed events, pointer events, keyboard events, gestures, focus management), animations and transitions (implicit transitions, keyframe animations, page transitions, easing functions), data templates and collections (DataTemplate, IDataTemplate, template selectors, virtualization, TreeDataGrid), cross-platform integration (IStorageProvider file dialogs, clipboard, drag-drop, system tray, NativeMenu, window customization, avares:// assets), desktop UI design principles (information density, navigation patterns, typography, color/theming, Fluent Design, form layout, geometry), interaction and UX patterns (keyboard-first design, accessibility, progress indicators, notifications, confirmation/undo), validation and error handling (INotifyDataErrorInfo, ObservableValidator, DataAnnotations, global exception handling), testing and deployment (ViewModel unit testing, Avalonia.Headless, publishing, MSIX, DMG, AppImage, Velopack, NativeAOT), and desktop observability (Serilog for desktop, crash reporting, performance diagnostics, offline telemetry). Invoke when: creating or modifying .axaml files, building desktop applications with Avalonia or WPF, implementing MVVM patterns for desktop, configuring Avalonia styling or theming, working with Avalonia controls or custom controls, setting up cross-platform desktop deployment, debugging Avalonia-specific issues (binding errors, styling not applying, platform differences), designing desktop UI layouts, or when the user asks about Avalonia or WPF patterns.