Adaptive Layout

Adapt to the space you are given, never to the device you think you are on. A phone in a foldable's front display, a resized desktop window, and a tablet in split-screen all defeat Platform.isX checks — but they all report their real constraints. Branch on width, not hardware.

Read the reference for the task at hand:

• references/window-size-classes.md — the Material 3 window size-class breakpoints as a shared vocabulary, the WindowSizeClass enum pattern, MediaQuery.sizeOf/paddingOf/viewInsetsOf vs .of, readable max-width, SafeArea and display cutouts, keyboard insets, orientation, foldable hinge awareness.
• references/list-detail-and-navigation.md — single-pane-navigate vs side-by-side two-pane, choosing the navigation affordance by width and coordinating with the go_router shell, keeping selection state in a Notifier so both panes agree.

Run scripts/check_adaptive.sh before a PR.

Non-negotiable rules

1. Adapt by constraints/size, never by device or platform. No Platform.isAndroid/Platform.isIOS/kIsWeb to pick a layout. Use LayoutBuilder (local box constraints) or MediaQuery.sizeOf(context) (window size). WHY: a resized window, split-screen, and foldable all break device checks; constraints are always true.
2. Use the Material 3 window size classes as the breakpoint vocabulary. Compact <600, medium 600–840, expanded 840–1200, large 1200–1600, extra-large ≥1600 (logical px width). WHY: these are STANDARD structural breakpoints (not design tokens); one shared enum keeps every screen's breakpoints identical.
3. Read the narrowest MediaQuery aspect: sizeOf / paddingOf / viewInsetsOf / viewPaddingOf, not MediaQuery.of(context). WHY: .of subscribes the widget to EVERY MediaQuery change (keyboard, rotation, text scale); the aspect getters rebuild only when that one field changes — see flutter-performance.
4. Pick the navigation affordance by width, not by a per-screen guess. NavigationBar (compact) → NavigationRail (medium/expanded) → NavigationDrawer (large). Decide once in the shell. WHY: mixing affordances across screens disorients; one width→affordance map is consistent by construction. Coordinate with navigation-and-routing's StatefulShellRoute.
5. List-detail collapses to one pane on compact and splits on expanded. Compact: the list navigates to a detail route. Expanded: list and detail sit side-by-side; selection is shared state. WHY: two panes on a phone are unreadable; one pane on a desktop wastes 70% of the width.
6. Never lock orientation (SystemChrome.setPreferredOrientations to force portrait). WHY: it breaks tablets, foldables, and accessibility mounts. Let the size classes handle both orientations.
7. Constrain readable content with a max width; use Flexible/Expanded/FractionallySizedBox, not fixed pixel widths, for top-level regions. WHY: full-width body text on a wide screen is unreadable (~40–75 chars/line is legible); fixed region widths overflow or leave dead space.
8. Wrap edge content in SafeArea and respect display cutouts + keyboard insets. Read cutouts/system bars via MediaQuery.paddingOf/viewPaddingOf; the keyboard via MediaQuery.viewInsetsOf. WHY: notches, punch-holes, and the on-screen keyboard occlude content that ignores insets.
9. Never assume a fixed cell/row height. Large text scale can double a cell's height; let content size itself. WHY: a hardcoded SizedBox(height: 48) clips at 200% text scale — see accessibility-as-code and widget-composition.
10. Verify across sizes with a golden matrix, not one device. Snapshot each adaptive screen at compact/medium/expanded (+ largest text, LTR/RTL). WHY: adaptive bugs only appear at the breakpoint you did not open — see widget-golden-and-a11y-testing.

Size classes as one shared enum

Define the breakpoints once; every screen reads the same map.

enum WindowSizeClass { compact, medium, expanded, large, extraLarge }

WindowSizeClass windowSizeClassFor(double width) => switch (width) {
      < 600 => WindowSizeClass.compact,
      < 840 => WindowSizeClass.medium,
      < 1200 => WindowSizeClass.expanded,
      < 1600 => WindowSizeClass.large,
      _ => WindowSizeClass.extraLarge,
    };

// In a widget: rebuilds only when the window SIZE changes.
final sizeClass = windowSizeClassFor(MediaQuery.sizeOf(context).width);

Prefer LayoutBuilder when the decision depends on the LOCAL box (a widget inside a split view, a card grid), and MediaQuery.sizeOf when it depends on the whole window (the top-level shell).

// Local constraints drive a card grid's column count.
LayoutBuilder(
  builder: (context, constraints) {
    final columns = constraints.maxWidth ~/ 280; // min card width, structural
    return GridView.count(crossAxisCount: columns.clamp(1, 4), children: cards);
  },
);

Readable width and flexible regions

// Center and cap body text to a legible measure; never full-bleed on desktop.
Center(
  child: ConstrainedBox(
    constraints: const BoxConstraints(maxWidth: 720), // structural readable cap
    child: article,
  ),
);

// Split a wide body proportionally, not with magic fixed widths.
Row(
  children: [
    Expanded(flex: 2, child: primaryPane),
    Expanded(flex: 3, child: secondaryPane),
  ],
);

Insets, cutouts, keyboard, foldables

// Keyboard height: rebuilds only when the keyboard shows/hides.
final keyboard = MediaQuery.viewInsetsOf(context).bottom;

// Notch/system-bar padding without subscribing to text-scale/rotation changes.
final safeTop = MediaQuery.paddingOf(context).top;

// Foldable: a vertical fold/hinge spanning the full height => split around it.
final seam = MediaQuery.displayFeaturesOf(context)
    .where((f) =>
        (f.type == DisplayFeatureType.fold ||
            f.type == DisplayFeatureType.hinge) &&
        f.bounds.top == 0 && // starts at the top edge...
        f.bounds.width < f.bounds.height) // ...taller than wide => vertical
    .firstOrNull; // firstOrNull: import 'package:collection/collection.dart';

Wrap the outermost interactive region in SafeArea; opt specific edges out (bottom: false) when a NavigationBar or scrolling body should reach the edge.

Navigation affordance by width

// One place decides the shell chrome; screens stay affordance-agnostic.
Widget shellFor(WindowSizeClass sc, {required Widget body, required int index}) {
  return switch (sc) {
    WindowSizeClass.compact =>
      Scaffold(body: body, bottomNavigationBar: _NavBar(index: index)),
    WindowSizeClass.medium || WindowSizeClass.expanded =>
      Scaffold(body: Row(children: [_Rail(index: index), Expanded(child: body)])),
    WindowSizeClass.large || WindowSizeClass.extraLarge =>
      Scaffold(body: Row(children: [_Drawer(index: index), Expanded(child: body)])),
  };
}

See examples/adaptive_scaffold.dart for the full shell and examples/list_detail_pane.dart for the two-pane pattern.

Anti-patterns

• if (Platform.isIOS) ... else ... to choose a layout. Branch on MediaQuery.sizeOf(context).width.
• MediaQuery.of(context).size.width — subscribes to every MediaQuery change. Use MediaQuery.sizeOf(context).
• SystemChrome.setPreferredOrientations([DeviceOrientation.portraitUp]) to dodge landscape layout. Support both.
• Container(width: 375) or SizedBox(width: 320) sizing a top-level pane. Use Expanded/Flexible/FractionallySizedBox/ConstrainedBox.
• A bool isTablet = MediaQuery.of(context).size.shortestSide > 600 flag threaded everywhere. Compute a WindowSizeClass from width at the shell.
• Two panes shown on compact width, forcing horizontal scroll. Collapse to one pane and navigate.
• Full-width paragraphs on desktop with no max-width. Cap the measure.
• Assuming the keyboard is hidden — content jumps under it. Pad by MediaQuery.viewInsetsOf(context).bottom.

Definition of done

• No Platform.is*/kIsWeb/dart:io used to select a layout (platform-specific plugins are fine).
• Breakpoints come from the shared WindowSizeClass helper, not ad-hoc numbers scattered per screen.
• MediaQuery is read via sizeOf/paddingOf/viewInsetsOf/viewPaddingOf, not .of.
• Navigation affordance is chosen by width in ONE place (the shell), coordinated with navigation-and-routing.
• List-detail screens collapse to one pane on compact and split on expanded, sharing selection state.
• No fixed pixel widths on top-level regions; readable content is max-width capped.
• Orientation is not locked; edge content uses SafeArea; keyboard insets are respected.
• Golden matrix covers compact/medium/expanded (+ largest text, RTL).
• scripts/check_adaptive.sh passes.

Related skills

• widget-composition — structural layout primitives, computed sizing, edge-to-edge vs SafeArea, never-fixed cell heights.
• navigation-and-routing — the StatefulShellRoute this shell renders; width picks the chrome, routes pick the branch.
• flutter-performance — why sizeOf/select beat .of for rebuild scope.
• accessibility-as-code — large text scale interacts with every adaptive cell.
• widget-golden-and-a11y-testing — the device × text-scale × RTL golden matrix that proves adaptivity.
• state-management-riverpod — where shared list-detail selection lives.

References

• Material 3 layout / window size classes: https://m3.material.io/foundations/layout/applying-layout/window-size-classes
• Adaptive & responsive design (Flutter): https://docs.flutter.dev/ui/adaptive-responsive
• MediaQuery API (aspect getters): https://api.flutter.dev/flutter/widgets/MediaQuery-class.html
• LayoutBuilder: https://api.flutter.dev/flutter/widgets/LayoutBuilder-class.html
• NavigationRail: https://api.flutter.dev/flutter/material/NavigationRail-class.html
• DisplayFeature / foldables: https://api.flutter.dev/flutter/dart-ui/DisplayFeature-class.html