Reactor Design

Windows 11 Design for Reactor

Author, review, and fix Reactor UI code following Windows 11 design system rules.

Reactor is a functional UI framework for WinUI 3 that builds UI entirely in C# — no XAML, no data binding, no ViewModels. UI is described with immutable Element records, composed via bare factory methods imported with using static Microsoft.UI.Reactor.Factories (TextBlock(...), VStack(...), etc.), and updated through a React-style reconciler with hooks (UseState, UseEffect, etc.).

Controlled prop note: when design examples use inputs such as TextBox(value, setValue) or Slider(value, ...), the factory keeps the simple call-site shape. Direct element-record reads of controlled props now return Optional<T>; use .GetValueOrDefault(...) or .Value. See migration/050-optional-t.md.

This skill translates the Windows 11 design language into Reactor's C# projection so that apps built with Reactor look, feel, and behave like first-class Windows 11 applications.

Workflow

1. Author new Reactor UI using the rules below.
2. Review a PR using the checklist at the end of this file.
3. Fix feedback by mapping issues to the specific rule and applying the correct pattern.
4. Verify changes using the testing guidance.

Quick Scan Checklist

Check these areas early in every review:

• Theme tokens used for all colors — no hardcoded hex strings for themed surfaces.
• High Contrast works — no opacity, no accent colors, only system color brushes.
• Typography uses Heading(), SubHeading(), Caption(), or WinUI style tokens — not raw FontSize/FontWeight.
• Layout values use the 4px grid.
• Text scaling and localization are safe — no fixed heights on text containers.
• Shadows have elevation (Translation(0, 0, 32)).
• Acrylic surfaces use correct background + border pairings.
• AutomationName() set on icon-only controls.
• Keys set on list items for stable reconciliation.

Core Rules (Always Apply)

1. Theming Is the Primary Lens

Use Theme.* tokens for all colors and brushes. Never hardcode hex colors for themed surfaces.

// Correct: theme tokens
TextBlock("Hello").Foreground(Theme.PrimaryText)
Border(child).Background(Theme.CardBackground)
Button("Action").Background(Theme.Accent)

// Wrong: hardcoded colors on themed surfaces
TextBlock("Hello").Foreground("#000000")
Border(child).Background("#FFFFFF")

Hardcoded colors are acceptable only for:

• One-off decorative elements that intentionally ignore theming (e.g., brand logos)
• Explicit hit-test targets (Background("#00000000") for transparent hit areas)

Theme Token Reference

⚠️ Theme.Error, Theme.Success, Theme.Warning, Theme.ErrorText do NOT exist. Use Theme.SystemCritical (red/error), Theme.SystemSuccess (green), Theme.SystemCaution (yellow).

Text:

Accent fill:

Control fill:

Surfaces & backgrounds:

Stroke & border:

System signal:

System signal backgrounds:

For any WinUI resource not exposed as a named token, use Theme.Ref("ResourceKeyBrush").

Custom Theme Resource References

// Reference any WinUI resource by key
Border(child).Background(Theme.Ref("AcrylicBackgroundFillColorDefaultBrush"))
Border(child).WithBorder(Theme.Ref("SurfaceStrokeColorFlyoutBrush"), 1)

Per-Subtree Theme Override

Force a subtree to a specific theme variant:

// Sidebar always renders in dark theme
VStack(sidebarContent).RequestedTheme(ElementTheme.Dark)

See theme-aware-resources.md for full token list and pairing rules.

2. Lightweight Styling (Per-Control Resource Overrides)

Override control template resources using .Resources() to customize visual states without replacing the entire template:

// Correct: override button resources for a subtle button
Button("Action", onClick).Resources(r => r
    .Set("ButtonBackground", Theme.SubtleFill)
    .Set("ButtonBackgroundPointerOver", Theme.Ref("SubtleFillColorSecondaryBrush"))
    .Set("ButtonBackgroundPressed", Theme.Ref("SubtleFillColorTertiaryBrush"))
    .Set("ButtonBorderBrush", Theme.SubtleFill)
    .Set("ButtonBorderBrushPointerOver", Theme.SubtleFill)
    .Set("ButtonBorderBrushPressed", Theme.SubtleFill))

// Wrong: setting Background directly — loses hover/pressed/disabled states
Button("Action", onClick).Background(Theme.SubtleFill)

Resource keys target the SolidColorBrush resource (ending in Brush), not the Color.

// Correct
.Set("ButtonBackground", Theme.Ref("ControlFillColorDefaultBrush"))

// Wrong — Color, not Brush
.Set("ButtonBackground", Theme.Ref("ControlFillColorDefault"))

3. High Contrast Rules (Strict)

High Contrast users rely on a fixed set of 8 system color brushes. Your UI must work with these constraints.

Allowed HC system brushes:

HC color pairings:

Rules:

• No hardcoded colors in HC mode.
• No opacity on elements or brushes in HC — encode translucency in the alpha channel for Light/Dark only.
• No accent colors or regular WinUI brushes in HC.
• No gradient animations in HC — use a single system brush.
• Use 2px border thickness for flyouts, dialogs, and cards in HC.
• No partial theme updates — when changing Light/Dark visual resources via .Resources(), include matching HC-safe values in the same change. Don't leave HC untested.
• Interactive containers in HC need a highlight border — for clickable cards or list items, add a SystemColorHighlightColor border in HC to indicate interactivity.
• Empty HC dictionary is valid — when .Resources() overrides target only Light/Dark and WinUI defaults already satisfy accessibility, you don't need to add HC-specific overrides.
• Set HighContrastAdjustment at app level to prevent system overrides:

  Application.Current.HighContrastAdjustment = ApplicationHighContrastAdjustment.None;
  

Using Theme.* tokens correctly means HC usually "just works" because WinUI resolves them to appropriate system colors. Test to verify.

4. Typography Must Use Semantic Styles

Use the predefined text factories or WinUI style tokens. Never set FontSize and FontWeight directly for standard UI text.

Reactor text factories:

The Title/Subtitle/Body/BodyStrong/BodyLarge/TitleLarge/Display factories map 1:1 to WinUI's named TextBlock styles (Spec 039 §17.6). Prefer them when matching WinUI design specs; the Heading/SubHeading factories are the older Reactor presets and remain valid.

Font weight helpers:

TextBlock("Important label").SemiBold()
TextBlock("Page Title").Bold()

WinUI type ramp (via .ApplyStyle()):

For sizes not covered by the factories, use .ApplyStyle() to apply WinUI text block styles. This sets size, weight, line height, and optical sizing in one call:

// Preferred: .ApplyStyle() for WinUI styles
TextBlock("Title").ApplyStyle("TitleTextBlockStyle")
TextBlock("Subtitle").ApplyStyle("SubtitleTextBlockStyle")
TextBlock("Body Strong").ApplyStyle("BodyStrongTextBlockStyle")
TextBlock("Caption").ApplyStyle("CaptionTextBlockStyle")

// Also correct: use the Reactor factories for common sizes
Heading("Page Title")
SubHeading("Section")
Caption("Fine print")

// Every WinUI type-ramp style has a factory except CaptionTextBlockStyle —
// Caption() is a size-only preset, so use .ApplyStyle("CaptionTextBlockStyle")
// when you need that style's line height and other setters
BodyLarge("Prominent text")
TitleLarge("Feature title")
Display("Hero")

// For any other named style, use .ApplyStyle(): it reports an unresolved key
// instead of throwing. Never assign Application.Current.Resources[...] through
// .Set() — that indexer throws out of the mount action on a misspelled key.
TextBlock("Custom").ApplyStyle("MyAppTextBlockStyle")

// Wrong: raw font properties for standard UI text
TextBlock("Title").FontSize(28).FontWeight(new FontWeight(700))

Rules:

• Use SemiBold (600), never Bold (700) for emphasis — except Heading() which intentionally uses 700 for page titles.
• Minimum font size: 12px. Anything smaller makes complex scripts unreadable.
• Use SymbolThemeFontFamily for icon fonts, applied with the .FontFamily(...) modifier (not .Set, which re-runs every update and is never unwound):

  TextBlock("\uE710")
      .FontFamily((FontFamily)Application.Current.Resources["SymbolThemeFontFamily"])
  

  Where no live Application.Current is available, the explicit "Segoe Fluent Icons, Segoe MDL2 Assets" stack is an acceptable static fallback. Never a bare "Segoe MDL2 Assets" (Windows 10 legacy) or a bare "Segoe Fluent Icons" (drops the Windows 10 tail).
• When icons and text are paired, top-align both in wrapping scenarios to prevent visual drift at larger text scales.
• TextWrapping: NoWrap is the default — use TextWrapping.Wrap or TextWrapping.WrapWholeWords when text should flow to multiple lines. Choose WrapWholeWords for body text to avoid mid-word breaks.
• Smart tooltips for trimmed text: When text is trimmed with TextTrimming, add a tooltip that only appears when the text is actually trimmed:

  TextBlock(longText)
      .TextTrimming(TextTrimming.CharacterEllipsis)
      .ToolTip(longText)
  

See typography-and-colors.md for the full type ramp and color token list.

5. Layout and Scaling

4px Grid

Use multiples of 4 for all margins, padding, and sizing values. The spacing scale is built on a 4px base unit:

// Correct: multiples of 4
VStack(8, children).Padding(16)
Border(child).Margin(12).Padding(8)
HStack(4, items)

// Flex containers take FlexPadding, NOT Padding. `.Padding()` silently has no
// effect on a FlexElement (FlexRow / FlexColumn / Flex) — `mur check` reports
// this as REACTOR_MOD_003. Don't add a Border solely to get padding; a Border is
// still right when you also need its background, corner radius, or border brush.
FlexColumn(children).FlexPadding(16)
FlexRow(items).FlexPadding(horizontal: 16, vertical: 8)

// Wrong: odd values cause blurry rendering at fractional scales
VStack(5, children).Padding(15)
Border(child).Margin(3)

// Wrong: no effect, and costs a build-check cycle
FlexColumn(children).Padding(16)

Margin vs Padding

Margin pushes an element away from its neighbors. It works on every Reactor element.

Padding adds space between a container's edge and its content. It works on Border, Control-based elements (Button, TextBox, etc.), Grid, StackPanel-based layout panels (VStack, HStack), RelativePanel, and TextBlock-based text (Text, TextBlock, and the typography factories). Image does not support .Padding() — wrap it in a Border if you need inner padding. Grid, VStack/HStack, and RelativePanel also accept .CornerRadius() directly.

// Margin — works on ALL elements
TextBlock("Hello").Margin(8)
VStack(children).Margin(16)
Border(child).Margin(12)

// Padding — Border, Control, Grid, StackPanel, RelativePanel and TextBlock
Border(child).Padding(16)          // ✓ works
Button("Go").Padding(12)           // ✓ works
Grid(columns, rows, cells).Padding(16)  // ✓ works
VStack(8, items).Padding(16)       // ✓ works
TextBlock("Hello").Padding(8)      // ✓ works

Both .Margin() and .Padding() accept four overloads:

// Uniform — same on all sides
element.Margin(16)

// Horizontal, Vertical
element.Margin(horizontal: 24, vertical: 8)

// Per-side: left, top, right, bottom
element.Margin(left: 4, top: 8, right: 16, bottom: 24)

// A Thickness you already have — useful when lifting a value out of a .Set(...)
element.Margin(new Thickness(68, 4, 40, 4))

Corner Radius

Use system values — ControlCornerRadius (4px) for controls and OverlayCornerRadius (8px) for overlays.

WinUI provides two corner radius theme resources. Use ThemeResource.CornerRadius() to resolve the system values at render time, ensuring your UI adapts if these values are customized:

// Preferred: resolve system values at render time
var controlRadius = ThemeResource.CornerRadius("ControlCornerRadius");
var overlayRadius = ThemeResource.CornerRadius("OverlayCornerRadius");

// Apply to elements
Border(child).CornerRadius(controlRadius.TopLeft)   // controls, buttons, cards
Border(dialog).CornerRadius(overlayRadius.TopLeft)   // dialogs, flyouts, menus

// Also acceptable: hardcoded system values
Border(child).CornerRadius(4)   // ControlCornerRadius equivalent
Border(child).CornerRadius(8)   // OverlayCornerRadius equivalent

// Selective rounding (top corners only)
Border(child).CornerRadius(8, 8, 0, 0)

// Wrong: non-standard radii (even if from Figma)
Border(child).CornerRadius(3)
Border(child).CornerRadius(6)

Mixed radii for nested surfaces: When nesting controls inside overlay containers, the outer container uses OverlayCornerRadius while inner controls use ControlCornerRadius:

var cr = ThemeResource.CornerRadius("ControlCornerRadius");
var or = ThemeResource.CornerRadius("OverlayCornerRadius");

// Dialog with control-radius inner elements
Border(
    VStack(12,
        TextBox("", placeholderText: "Username").CornerRadius(cr.TopLeft),
        Button("Sign In", onClick)
            .Background(Theme.Accent)
            .CornerRadius(cr.TopLeft)
    ).Margin(24)
)
.Background(Theme.LayerFill)
.WithBorder(Theme.SurfaceStroke)
.CornerRadius(or.TopLeft)  // outer overlay radius

Sizing

// Correct: MinHeight for flexible sizing
Button("Action").MinHeight(40)
VStack(children).MinWidth(200)

// Wrong: fixed Height clips at larger text scales
Button("Action").Height(32)
TextBox(text, setText).Height(30)

• Prefer MinHeight/MinWidth over fixed Height/Width.
• Avoid fixed widths on buttons — let content drive width.
• Buttons achieve correct height through padding, not explicit Height.

Container Choice

Pick the right container. Prefer FlexRow / FlexColumn as the default for linear layout — they follow CSS Flexbox semantics (grow, shrink, basis, gap, wrap, justify-content, align-items), which matches the mental model web- trained engineers and designers already have. VStack / HStack remain a good choice when you specifically want StackPanel's shrink-wrap cross-axis behavior or are porting from existing StackPanel code.

Remove wrapper containers (Border, Grid, FlexColumn, VStack) that exist only for nesting without contributing layout, styling, or semantic purpose.

Text trimming caveat: HStack (StackPanel) and FlexRow both give children unbounded width on the main axis, so TextTrimming never activates inside them. Use a Grid with a GridSize.Star() column. Note: GridSize.Auto also sizes to content and prevents trimming — always use GridSize.Star() for the column that contains trimmable text.

// Correct: Grid constrains width so trimming works
Grid(
    columns: [GridSize.Auto, GridSize.Star()],
    rows: [GridSize.Auto],
    Image(source).Size(32, 32).Grid(column: 0),
    TextBlock(title).TextTrimming(TextTrimming.CharacterEllipsis).Grid(column: 1))

// Wrong: TextTrimming never fires inside HStack
HStack(8,
    Image(source).Size(32, 32),
    TextBlock(title).TextTrimming(TextTrimming.CharacterEllipsis))

ScrollView Configuration

• Use Auto scrollbar visibility — scrollbar appears only when content overflows.
• Set HorizontalContentAlignment = Stretch on the ScrollView to prevent content from collapsing.
• Only the content area should scroll — headers and action bars remain outside the ScrollView.

// Correct: header stays fixed, content scrolls
VStack(
    Heading("Page Title"),
    ScrollView(VStack(8, contentItems)))

Window Title Bar

Prefer TitleBar(...) as the top-of-window element for Reactor desktop apps where a title makes sense (most main windows, document shells, settings windows). It integrates with the Windows caption (drag region, system menu, min/max/close) and themes with the rest of the app, and it accepts inline Content and RightHeader for branding, document context, or small inline controls — which avoids the common mistake of stacking a custom header row below the system chrome and ending up with two visual title zones.

// Preferred: real Windows title bar with app title + subtitle + inline content
var titleBar = (TitleBar("MyApp") with
{
    Subtitle = currentDoc,
    Content = ModeSwitcher(mode, setMode),
    RightHeader = TextBlock($"{rowsLoaded:N0} rows").FontSize(12).Opacity(0.6),
}).Flex(shrink: 0);

return FlexColumn(titleBar, MainContent());

Small dialogs, embedded pop-outs, and tool windows that already have a distinct visual header don't need TitleBar — use it where you'd otherwise reach for a custom header row across the full window width.

Spacing

Use FlexColumn(...) with { RowGap = n } / FlexRow(...) with { ColumnGap = n } (or VStack(spacing, ...) / HStack(spacing, ...)) or Grid RowSpacing/ColumnSpacing — not spacer elements.

// Correct
VStack(8, TextBlock("A"), TextBlock("B"), TextBlock("C"))

// Wrong: spacer element for spacing
VStack(
    TextBlock("A"),
    Border(null).Height(8),  // Don't do this
    TextBlock("B"))

Shadows

ThemeShadow requires elevation to be visible. Add Translation(0, 0, 32) and ensure the parent has padding (12px) to prevent shadow clipping:

Border(
    ScrollView(VStack(16, content))
).Background(Theme.Ref("AcrylicBackgroundFillColorDefaultBrush"))
 .WithBorder(Theme.Ref("SurfaceStrokeColorFlyoutBrush"), 1)
 .CornerRadius(8)
 .Translation(0, 0, 32)
 .Set(b =>
 {
     b.BackgroundSizing = BackgroundSizing.InnerBorderEdge;
     b.Shadow = new ThemeShadow();
 })

See layout-and-scaling.md for full layout rules.

6. Data Flow and State

Reactor uses hooks, not MVVM data binding. Follow these patterns:

State-Driven UI (Preferred)

var (items, setItems) = UseState(new List<Item>());
var (filter, setFilter) = UseState("");

var filtered = UseMemo(() =>
    items.Where(i => i.Name.Contains(filter, StringComparison.OrdinalIgnoreCase)).ToList(),
    items, filter);

return VStack(
    TextBox(filter, setFilter, placeholderText: "Filter..."),
    VStack(filtered.Select(item =>
        TextBlock(item.Name).WithKey(item.Id)
    ).ToArray()));

UseObservable for External Models

When integrating with existing INotifyPropertyChanged objects:

var model = UseObservable(externalModel);

return VStack(
    TextBlock(model.Title),
    Slider(model.Volume, 0, 100, v => model.Volume = v));

Hook Rules (Critical)

1. Same order every render — no hooks inside if blocks, no hooks in variable-length loops.
2. Only call from Render() — or from within a function component body.
3. Use UseCallback for stable references — handlers passed to children should be memoized.

// Correct: hooks at top level, unconditional
var (count, setCount) = UseState(0);
var (name, setName) = UseState("");
var increment = UseCallback(() => setCount(count + 1), count);

// Wrong: conditional hook
if (showCounter)
{
    var (count, setCount) = UseState(0);  // BREAKS hook ordering
}

7. Accessibility

Tier 1 Modifiers (Every Control)

These modifiers are zero-cost — apply them by default:

// Icon-only buttons MUST have AutomationName
Button(Content: Image(iconSource), onClick)
    .AutomationName("Close dialog")

// Headings for screen reader navigation
Heading("Settings").HeadingLevel(AutomationHeadingLevel.Level1)
SubHeading("General").HeadingLevel(AutomationHeadingLevel.Level2)

// Tab order and access keys
Button("Save", onSave).IsTabStop(true).TabIndex(0).AccessKey("S")

Tier 2 Modifiers (Lazy-Allocated)

These allocate an AutomationProperties peer only when set:

TextBox(name, setName)
    .HelpText("Enter your full legal name")
    .Required()

// Hide decorative elements from screen readers
Image(decorativeSource).AccessibilityHidden()

// Position-in-set for screen readers on list items
items.Select((item, i) =>
    TextBlock(item.Name)
        .PositionInSet(i + 1, items.Count)
        .WithKey(item.Id))

Navigation Landmarks

Landmarks let screen reader users jump between page regions:

NavigationView(navItems, content)
    .Landmark(AutomationLandmarkType.Navigation)

FlexColumn(mainContent)
    .Landmark(AutomationLandmarkType.Main)

VStack(searchControls)
    .Landmark(AutomationLandmarkType.Search)

VStack(formFields)
    .Landmark(AutomationLandmarkType.Form)

Heading Hierarchy

Maintain a logical heading structure for screen reader navigation:

// Good: proper hierarchy
Heading("Settings").HeadingLevel(AutomationHeadingLevel.Level1)
SubHeading("General").HeadingLevel(AutomationHeadingLevel.Level2)
SubHeading("Advanced").HeadingLevel(AutomationHeadingLevel.Level2)

// Bad: skipping levels
Heading("Settings").HeadingLevel(AutomationHeadingLevel.Level1)
Caption("Detail").HeadingLevel(AutomationHeadingLevel.Level4) // skipped 2 and 3

Focus Trapping (Dialogs / Modals)

Use UseFocusTrap to keep focus inside a modal:

var trap = this.UseFocusTrap(isActive: true);

return Border(
    VStack(12,
        Heading("Confirm"),
        TextBlock("Are you sure?"),
        HStack(8,
            Button("Cancel", onCancel),
            Button("Confirm", onConfirm))
    ).Padding(24)
).FocusTrap(trap);  // Tab cycles within this container

UseFocusTrap(isActive) returns a FocusTrapHandle. Apply it with .FocusTrap(handle). Focus is trapped while isActive is true.

UseAnnounce (Live Announcements)

For dynamic status updates that aren't tied to a visible element:

var announce = UseAnnounce();

// The announce region must be in the visual tree
return VStack(12,
    announce.Region,  // invisible — just hosts the live region
    Button("Save", async () =>
    {
        await Save();
        announce.Announce("Document saved successfully");
    }),
    Button("Error demo", () =>
        announce.Announce("Upload failed — check your connection", assertive: true))
);

Important: announce.Region must be in the visual tree or announcements are silently lost. Place it once near the root of your page.

SemanticPanel (Custom Roles)

For controls that don't map to standard WinUI automation peers:

Border(child).Semantics(
    role: "Slider",
    value: $"{percent}%",
    rangeMin: 0,
    rangeMax: 100,
    rangeValue: percent);

AccessibilityScanner (Automated Testing)

Run WCAG checks programmatically:

var results = AccessibilityScanner.Scan(rootElement);

// Built-in checks include:
// - Missing AutomationName on interactive controls
// - Missing HeadingLevel on heading text
// - Color contrast ratio < 4.5:1
// - Touch target size < 44x44
// - Missing landmark regions
// - Tab order gaps
// - Missing PositionInSet on list items
// - AccessKey conflicts

AccessibilityScanner.ExportJson(results, "a11y-report.json");

Roslyn Analyzers

Three compile-time analyzers catch accessibility issues:

These run as warnings by default. Promote to errors in CI:

<WarningsAsErrors>REACTOR_A11Y_001;REACTOR_A11Y_002;REACTOR_A11Y_003</WarningsAsErrors>

Accessible Forms Pattern

Combine validation with accessibility:

var validation = this.UseValidationContext();
var (email, setEmail) = UseState("");

return FormField(
    TextBox(email, setEmail)
        .Validate("email", email, Validate.Required(), Validate.Email())
        .Required()
        .HelpText("We'll send a confirmation to this address"),
    label: "Email",
    required: true,
    showWhen: ShowWhen.WhenTouched)
.Landmark(AutomationLandmarkType.Form);

FormField automatically wires AutomationName from the label and associates error messages with the input via DescribedBy.

Accessibility Rules

• Set AutomationName on every control without visible text.
• Maintain heading hierarchy — don't skip levels.
• Use landmarks on major page regions.
• Focus-trap all dialogs and modals.
• Place announce.Region in the tree before calling .Announce().
• Test with Narrator + keyboard-only navigation.
• Test Light, Dark, and High Contrast themes (especially NightSky).
• Test at 100%, 150%, 200%, 250% display scaling.
• Test with maximum text scaling (Settings > Accessibility > Text size).
• Hit-test targets for light-dismiss must be visible: Background("#00000000").
• Use DividerStrokeColorDefaultBrush for dividers — custom brushes with opacity break in HC.
• Enable Roslyn analyzers in CI to catch common issues at build time.

See code-review-checklist.md for the full accessibility checklist.

8. Acrylic Surface Pairings

Acrylic backgrounds have specific border pairings. Using the wrong combination produces incorrect visuals.

// Correct: flyout acrylic pairing
Border(content)
    .Background(Theme.Ref("AcrylicBackgroundFillColorDefaultBrush"))
    .WithBorder(Theme.Ref("SurfaceStrokeColorFlyoutBrush"), 1)
    .CornerRadius(8)
    .Translation(0, 0, 32)
    .Set(b =>
    {
        b.BackgroundSizing = BackgroundSizing.InnerBorderEdge;
        b.Shadow = new ThemeShadow();
    })

BackgroundSizing = InnerBorderEdge is required on any bordered acrylic surface. It prevents the background from bleeding through the border edge. Always set it on acrylic containers with a border.

• Overlays on acrylic use LayerOnAcrylicFillColorDefaultBrush.
• Keep one acrylic layer per visual surface to avoid stacked-material artifacts.

Window Backdrops (Mica / Acrylic)

For window-level material (Mica/Acrylic showing through the title bar and chrome), use the .Backdrop(BackdropKind) modifier on the root element — not a brush. The modifier walks up to the host's Window and assigns the right SystemBackdrop:

return Grid([GridSize.Star()], [GridSize.Auto, GridSize.Star()],
    titleBar.Grid(row: 0),
    content.Grid(row: 1)
).Backdrop(BackdropKind.Mica);

Available kinds: BackdropKind.None, Mica, MicaAlt, DesktopAcrylic, AcrylicThin. On ReactorHostControl (windowless host) the modifier no-ops cleanly. For Mica to show through, the root element must not paint an opaque background — drop Theme.SolidBackground from the root when applying a backdrop. Spec 033 §6.

Flyout Surface Pattern

Flyout/popup surfaces should follow a standard elevation pattern:

Border(
    ScrollView(VStack(8, flyoutContent))
)
.Background(Theme.Ref("FlyoutPresenterBackground"))
.WithBorder(Theme.Ref("FlyoutBorderThemeBrush"), 1)
.CornerRadius(8)
.Translation(0, 0, 32)
.Set(b =>
{
    b.BackgroundSizing = BackgroundSizing.InnerBorderEdge;
    b.Shadow = new ThemeShadow();
})

Use FlyoutPresenterBackground and FlyoutBorderThemeBrush for standard popup surfaces. Use the explicit acrylic resource pairings (above) only when building custom surfaces that don't use WinUI's flyout presenter resources.

9. Animation and Motion

Implicit Transitions (Recommended)

Animate property changes smoothly — the framework handles interpolation:

// Opacity fade
Border(child)
    .OpacityTransition()
    .Opacity(isVisible ? 1.0 : 0.0)

// Background color crossfade
VStack(children)
    .BackgroundTransition()
    .Background(isActive ? Theme.Accent : Theme.ControlFill)

// Scale bounce
Border(child)
    .ScaleTransition()
    .Scale(isPressed ? 0.95f : 1.0f)

Layout Animations

Animate children being added, removed, or repositioned:

VStack(items.Select(item =>
    TextBlock(item.Name).WithKey(item.Id)
).ToArray()).LayoutAnimation()

Element Enter/Exit Transitions

// Fade + slide from bottom on mount
Border(child).WithTransitions(
    Transition.Fade,
    Transition.Slide(Edge.Bottom))

Rules:

• Use ThemeShadow instead of composition drop shadows.
• Avoid gradient animations in High Contrast.
• Use BrushTransition (via BackgroundTransition()) for smooth color changes.
• Always set Translation(0, 0, 32) when using ThemeShadow.

10. Reconciliation and Performance

Keys for Lists

Always set .WithKey() on items in dynamic lists. Without keys, the reconciler matches by position, causing unnecessary re-mounts on insert/reorder:

// Correct: keyed children — stable identity
VStack(items.Select(item =>
    HStack(8,
        Image(item.Avatar).Size(32, 32),
        TextBlock(item.Name)
    ).WithKey(item.Id)
).ToArray())

// Wrong: unkeyed — insert at index 0 re-renders everything
VStack(items.Select(item =>
    HStack(8,
        Image(item.Avatar).Size(32, 32),
        TextBlock(item.Name)
    ).ToArray())

Memoize Expensive Computations

var sorted = UseMemo(() =>
    items.OrderBy(x => x.Name).ToList(),
    items);

var handler = UseCallback(() => save(count), count);

Avoid Deep Nesting

Flatten visual tree depth where possible. Use Border instead of single-child Grid/VStack.

// Correct: flat
Border(TextBlock("Hello")).Background(Theme.CardBackground).Padding(16)

// Even better: the Card(

…(truncated)