Add box-sizing CSS property support

packages/spark_css/CHANGELOG.md

@@ -12,6 +12,7 @@
 - **Feat**: Added `CssAnimationDirection`, `CssAnimationFillMode`, `CssAnimationPlayState`, and `CssAnimationIterationCount` sealed classes for animation sub-properties. Reuses existing `CssDuration` and `CssTimingFunction` from `CssTransition`.
 - **Feat**: Added `CssBorderCollapse` sealed class for `border-collapse` property with `.separate`, `.collapse` keywords, plus `.variable()`, `.raw()`, and `.global()` escape hatches.
 - **Feat**: Added `border-top-color`, `border-right-color`, `border-bottom-color`, and `border-left-color` CSS property support via `CssColor?` parameters in `Style.typed()`.
+- **Feat**: Added `CssBoxSizing` sealed class for `box-sizing` property with `.contentBox`, `.borderBox` keywords, plus `.variable()`, `.raw()`, and `.global()` escape hatches.
 
 ### Changed
 

packages/spark_css/lib/src/css_types/css_box_sizing.dart

@@ -0,0 +1,50 @@
+import 'css_value.dart';
+
+/// CSS box-sizing property values.
+sealed class CssBoxSizing implements CssValue {
+  const CssBoxSizing._();
+
+  static const CssBoxSizing contentBox = _CssBoxSizingKeyword('content-box');
+  static const CssBoxSizing borderBox = _CssBoxSizingKeyword('border-box');
+
+  /// CSS variable reference.
+  factory CssBoxSizing.variable(String varName) = _CssBoxSizingVariable;
+
+  /// Raw CSS value escape hatch.
+  factory CssBoxSizing.raw(String value) = _CssBoxSizingRaw;
+
+  /// Global keyword (inherit, initial, unset, revert).
+  factory CssBoxSizing.global(CssGlobal global) = _CssBoxSizingGlobal;
+}
+
+final class _CssBoxSizingKeyword extends CssBoxSizing {
+  final String keyword;
+  const _CssBoxSizingKeyword(this.keyword) : super._();
+
+  @override
+  String toCss() => keyword;
+}
+
+final class _CssBoxSizingVariable extends CssBoxSizing {
+  final String varName;
+  const _CssBoxSizingVariable(this.varName) : super._();
+
+  @override
+  String toCss() => 'var(--$varName)';
+}
+
+final class _CssBoxSizingRaw extends CssBoxSizing {
+  final String value;
+  const _CssBoxSizingRaw(this.value) : super._();
+
+  @override
+  String toCss() => value;
+}
+
+final class _CssBoxSizingGlobal extends CssBoxSizing {
+  final CssGlobal global;
+  const _CssBoxSizingGlobal(this.global) : super._();
+
+  @override
+  String toCss() => global.toCss();
+}

packages/spark_css/lib/src/css_types/css_types.dart

@@ -15,6 +15,7 @@ export 'css_border.dart';
 export 'css_border_collapse.dart';
 export 'css_border_radius.dart';
 export 'css_box_shadow.dart';
+export 'css_box_sizing.dart';
 export 'css_color.dart';
 export 'css_cursor.dart';
 export 'css_display.dart';

packages/spark_css/lib/src/style.dart

@@ -306,6 +306,7 @@ class Style implements CssStyle {
     CssZIndex? zIndex,
     CssCursor? cursor,
     CssBoxShadow? boxShadow,
+    CssBoxSizing? boxSizing,
     CssFilter? filter,
     CssFilter? backdropFilter,
     // Backgrounds
@@ -448,6 +449,7 @@ class Style implements CssStyle {
     if (zIndex != null) _properties['z-index'] = zIndex.toCss();
     if (cursor != null) _properties['cursor'] = cursor.toCss();
     if (boxShadow != null) _properties['box-shadow'] = boxShadow.toCss();
+    if (boxSizing != null) _properties['box-sizing'] = boxSizing.toCss();
     if (filter != null) _properties['filter'] = filter.toCss();
     if (backdropFilter != null) {
       _properties['backdrop-filter'] = backdropFilter.toCss();

packages/spark_css/test/css_box_sizing_test.dart

@@ -0,0 +1,24 @@
+import 'package:spark_css/spark_css.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('CssBoxSizing', () {
+    test('keywords output correct CSS', () {
+      expect(CssBoxSizing.contentBox.toCss(), equals('content-box'));
+      expect(CssBoxSizing.borderBox.toCss(), equals('border-box'));
+    });
+    test('variable outputs correct CSS', () {
+      expect(CssBoxSizing.variable('bs').toCss(), equals('var(--bs)'));
+    });
+    test('raw outputs value as-is', () {
+      expect(CssBoxSizing.raw('border-box').toCss(), equals('border-box'));
+    });
+    test('global outputs correct CSS', () {
+      expect(CssBoxSizing.global(CssGlobal.inherit).toCss(), equals('inherit'));
+    });
+    test('Style.typed integration', () {
+      final style = Style.typed(boxSizing: CssBoxSizing.borderBox);
+      expect(style.toCss(), contains('box-sizing: border-box;'));
+    });
+  });
+}