CSS engine: sync @layer and the user-preference media features

From PeachPDF v0.9.6 (upstream cc1a521, 607bcce).

- LayerRule / LayerStatementRule / ILayerRule: both @layer forms - the block
  form that groups rules and the statement form that only declares an order
- Seven new media features: prefers-color-scheme, prefers-reduced-motion,
  prefers-contrast, prefers-reduced-transparency, update, any-hover, any-pointer
- MediaFeature gains a Comparison property (derived from the min-/max- prefix
  or the range-syntax operator) plus AsLength/AsResolution/AsRatio accessors,
  which is what a media-query matcher needs to actually evaluate a feature

Layer *ordering* is not implemented - see the separate CssData change that
indexes a layer's contents as unlayered rules.

Author: jhaygood86

Source/HtmlRenderer/Core/CssEngine/Factories/MediaFeatureFactory.cs

@@ -63,6 +63,13 @@ internal sealed class MediaFeatureFactory
                 {FeatureNames.Hover, () => new HoverMediaFeature()},
                 {FeatureNames.InlineSize, () => new SizeMediaFeature(FeatureNames.InlineSize)},
                 {FeatureNames.BlockSize, () => new SizeMediaFeature(FeatureNames.BlockSize)},
+                {FeatureNames.PrefersColorScheme, () => new PrefersColorSchemeMediaFeature()},
+                {FeatureNames.PrefersReducedMotion, () => new PrefersReducedMotionMediaFeature()},
+                {FeatureNames.PrefersContrast, () => new PrefersContrastMediaFeature()},
+                {FeatureNames.PrefersReducedTransparency, () => new PrefersReducedTransparencyMediaFeature()},
+                {FeatureNames.Update, () => new UpdateMediaFeature()},
+                {FeatureNames.AnyHover, () => new AnyHoverMediaFeature()},
+                {FeatureNames.AnyPointer, () => new AnyPointerMediaFeature()},
             };
 
         #endregion

Source/HtmlRenderer/Core/CssEngine/MediaFeatures/AnyHoverMediaFeature.cs

@@ -0,0 +1,15 @@
+namespace TheArtOfDev.HtmlRenderer.Core.CssEngine
+{
+    /// <summary>
+    /// The <c>any-hover</c> media feature (Media Queries 4). Matched against the renderer's fixed
+    /// <c>none</c> stance (a static PDF has no hover-capable input) in <c>MediaQueryMatcher</c>.
+    /// </summary>
+    internal sealed class AnyHoverMediaFeature : MediaFeature
+    {
+        public AnyHoverMediaFeature() : base(FeatureNames.AnyHover)
+        {
+        }
+
+        internal override IValueConverter Converter => Converters.Any;
+    }
+}

Source/HtmlRenderer/Core/CssEngine/MediaFeatures/AnyPointerMediaFeature.cs

@@ -0,0 +1,15 @@
+namespace TheArtOfDev.HtmlRenderer.Core.CssEngine
+{
+    /// <summary>
+    /// The <c>any-pointer</c> media feature (Media Queries 4). Matched against the renderer's fixed
+    /// <c>none</c> stance (a static PDF has no pointing device) in <c>MediaQueryMatcher</c>.
+    /// </summary>
+    internal sealed class AnyPointerMediaFeature : MediaFeature
+    {
+        public AnyPointerMediaFeature() : base(FeatureNames.AnyPointer)
+        {
+        }
+
+        internal override IValueConverter Converter => Converters.Any;
+    }
+}

Source/HtmlRenderer/Core/CssEngine/MediaFeatures/MediaFeature.cs

@@ -1,7 +1,26 @@
 using System.IO;
+using System.Linq;
 
 namespace TheArtOfDev.HtmlRenderer.Core.CssEngine
 {
+    /// <summary>
+    /// How a media feature's stored value constrains the device value, unifying the <c>min-</c>/<c>max-</c>
+    /// name-prefix forms with the Media Queries 4 range operators (<c>&gt;=</c>/<c>&lt;=</c>/<c>&gt;</c>/<c>&lt;</c>).
+    /// </summary>
+    internal enum MediaFeatureComparison
+    {
+        /// <summary>Exact match (the <c>feature: value</c> form, or a boolean feature).</summary>
+        Exact,
+        /// <summary>Device value must be ≥ the feature value (<c>min-*</c> or <c>&gt;=</c>).</summary>
+        Minimum,
+        /// <summary>Device value must be ≤ the feature value (<c>max-*</c> or <c>&lt;=</c>).</summary>
+        Maximum,
+        /// <summary>Device value must be strictly &gt; the feature value.</summary>
+        GreaterThan,
+        /// <summary>Device value must be strictly &lt; the feature value.</summary>
+        LessThan
+    }
+
     internal abstract class MediaFeature : StylesheetNode, IMediaFeature
     {
         private TokenValue _tokenValue;
@@ -26,6 +45,49 @@ internal MediaFeature(string name)
 
         public bool HasValue => _tokenValue != null && _tokenValue.Count > 0;
 
+        /// <summary>
+        /// The comparison the stored value imposes on the device value, derived from the <c>min-</c>/
+        /// <c>max-</c> name prefix or, for the range syntax (<c>width &gt;= 48rem</c>), the parsed operator.
+        /// </summary>
+        internal MediaFeatureComparison Comparison
+        {
+            get
+            {
+                if (IsMinimum) return MediaFeatureComparison.Minimum;
+                if (IsMaximum) return MediaFeatureComparison.Maximum;
+
+                switch (_constraintDelimiter)
+                {
+                    case TokenType.GreaterThanOrEqual: return MediaFeatureComparison.Minimum;
+                    case TokenType.GreaterThan: return MediaFeatureComparison.GreaterThan;
+                    case TokenType.LessThanOrEqual: return MediaFeatureComparison.Maximum;
+                    case TokenType.LessThan: return MediaFeatureComparison.LessThan;
+                    default: return MediaFeatureComparison.Exact;
+                }
+            }
+        }
+
+        /// <summary>The stored value parsed as a <see cref="Length"/>, or <c>null</c> if absent/not a length.</summary>
+        internal Length? AsLength() => HasValue ? _tokenValue.ToLength() : null;
+
+        /// <summary>The stored value parsed as a <see cref="Resolution"/>, or <c>null</c> if absent/not a resolution.</summary>
+        internal Resolution? AsResolution() => HasValue ? _tokenValue.ToResolution() : null;
+
+        /// <summary>
+        /// The stored value parsed as a numeric ratio (<c>&lt;number&gt; [ / &lt;number&gt; ]?</c>), e.g.
+        /// <c>16/9</c>, or <c>null</c> if absent/unparseable.
+        /// </summary>
+        internal double? AsRatio()
+        {
+            if (!HasValue) return null;
+
+            var numbers = _tokenValue.OfType<NumberToken>().Select(token => (double)token.Value).ToList();
+
+            if (numbers.Count >= 2) return numbers[1] != 0 ? numbers[0] / numbers[1] : (double?)null;
+            if (numbers.Count == 1) return numbers[0] > 0 ? numbers[0] : (double?)null;
+            return null;
+        }
+
         public override void ToCss(TextWriter writer, IStyleFormatter formatter)
         {
             var constraintDelimiter = GetConstraintDelimiter();

Source/HtmlRenderer/Core/CssEngine/MediaFeatures/PrefersColorSchemeMediaFeature.cs

@@ -0,0 +1,16 @@
+namespace TheArtOfDev.HtmlRenderer.Core.CssEngine
+{
+    /// <summary>
+    /// The <c>prefers-color-scheme</c> media feature (Media Queries 5). The value is validated as any
+    /// identifier here; the actual <c>light</c>/<c>dark</c> match against the configured scheme happens
+    /// in <c>MediaQueryMatcher</c>.
+    /// </summary>
+    internal sealed class PrefersColorSchemeMediaFeature : MediaFeature
+    {
+        public PrefersColorSchemeMediaFeature() : base(FeatureNames.PrefersColorScheme)
+        {
+        }
+
+        internal override IValueConverter Converter => Converters.Any;
+    }
+}

Source/HtmlRenderer/Core/CssEngine/MediaFeatures/PrefersContrastMediaFeature.cs

@@ -0,0 +1,15 @@
+namespace TheArtOfDev.HtmlRenderer.Core.CssEngine
+{
+    /// <summary>
+    /// The <c>prefers-contrast</c> media feature (Media Queries 5). Matched against the renderer's fixed
+    /// <c>no-preference</c> stance in <c>MediaQueryMatcher</c>.
+    /// </summary>
+    internal sealed class PrefersContrastMediaFeature : MediaFeature
+    {
+        public PrefersContrastMediaFeature() : base(FeatureNames.PrefersContrast)
+        {
+        }
+
+        internal override IValueConverter Converter => Converters.Any;
+    }
+}

Source/HtmlRenderer/Core/CssEngine/MediaFeatures/PrefersReducedMotionMediaFeature.cs

@@ -0,0 +1,15 @@
+namespace TheArtOfDev.HtmlRenderer.Core.CssEngine
+{
+    /// <summary>
+    /// The <c>prefers-reduced-motion</c> media feature (Media Queries 5). Matched against the renderer's
+    /// fixed <c>reduce</c> stance (a static PDF has no motion) in <c>MediaQueryMatcher</c>.
+    /// </summary>
+    internal sealed class PrefersReducedMotionMediaFeature : MediaFeature
+    {
+        public PrefersReducedMotionMediaFeature() : base(FeatureNames.PrefersReducedMotion)
+        {
+        }
+
+        internal override IValueConverter Converter => Converters.Any;
+    }
+}

Source/HtmlRenderer/Core/CssEngine/MediaFeatures/PrefersReducedTransparencyMediaFeature.cs

@@ -0,0 +1,15 @@
+namespace TheArtOfDev.HtmlRenderer.Core.CssEngine
+{
+    /// <summary>
+    /// The <c>prefers-reduced-transparency</c> media feature (Media Queries 5). Matched against the
+    /// renderer's fixed <c>no-preference</c> stance in <c>MediaQueryMatcher</c>.
+    /// </summary>
+    internal sealed class PrefersReducedTransparencyMediaFeature : MediaFeature
+    {
+        public PrefersReducedTransparencyMediaFeature() : base(FeatureNames.PrefersReducedTransparency)
+        {
+        }
+
+        internal override IValueConverter Converter => Converters.Any;
+    }
+}

Source/HtmlRenderer/Core/CssEngine/MediaFeatures/UpdateMediaFeature.cs

@@ -0,0 +1,16 @@
+namespace TheArtOfDev.HtmlRenderer.Core.CssEngine
+{
+    /// <summary>
+    /// The <c>update</c> media feature (Media Queries 4): how frequently the output device can modify the
+    /// appearance of content. A printed/exported PDF cannot update once produced, so it is matched against
+    /// <c>none</c> in <c>MediaQueryMatcher</c>.
+    /// </summary>
+    internal sealed class UpdateMediaFeature : MediaFeature
+    {
+        public UpdateMediaFeature() : base(FeatureNames.Update)
+        {
+        }
+
+        internal override IValueConverter Converter => Converters.Any;
+    }
+}

Source/HtmlRenderer/Core/CssEngine/Rules/ILayerRule.cs

@@ -0,0 +1,13 @@
+namespace TheArtOfDev.HtmlRenderer.Core.CssEngine
+{
+    /// <summary>
+    /// The block form of the <c>@layer</c> cascade-layer at-rule (<c>@layer name { … }</c>, or an
+    /// anonymous <c>@layer { … }</c>). Groups the style rules that belong to a named cascade layer,
+    /// per <see href="https://www.w3.org/TR/css-cascade-5/#layering">CSS Cascade 5</see>.
+    /// </summary>
+    internal interface ILayerRule : IGroupingRule
+    {
+        /// <summary>The layer's (optionally dotted) name; empty for an anonymous layer.</summary>
+        string Name { get; }
+    }
+}