Do not over-apply sub-tables during contextual substitution list apply

src/SixLabors.Fonts/GlyphShapingData.cs

@@ -61,7 +61,10 @@ public GlyphShapingData(GlyphShapingData data, bool clearFeatures = false)
             this.Features.AddRange(data.Features);
         }
 
-        this.AppliedFeatures.AddRange(data.AppliedFeatures);
+        foreach (Tag feature in data.AppliedFeatures)
+        {
+            this.AppliedFeatures.Add(feature);
+        }
 
         this.Bounds = data.Bounds;
     }
@@ -124,7 +127,7 @@ public GlyphShapingData(GlyphShapingData data, bool clearFeatures = false)
     /// <summary>
     /// Gets or sets the collection of applied features.
     /// </summary>
-    public List<Tag> AppliedFeatures { get; set; } = new();
+    public HashSet<Tag> AppliedFeatures { get; set; } = new();
 
     /// <summary>
     /// Gets or sets the shaping bounds.

src/SixLabors.Fonts/Tables/AdvancedTypographic/AdvancedTypographicUtils.cs

@@ -51,7 +51,6 @@ public static bool ApplyLookupList(
         int index,
         int count)
     {
-        bool hasChanged = false;
         SkippingGlyphIterator iterator = new(fontMetrics, collection, index, lookupFlags);
         int currentCount = collection.Count;
 
@@ -62,7 +61,7 @@ public static bool ApplyLookupList(
             iterator.Index = index;
             iterator.Increment(sequenceIndex);
             GSub.LookupTable lookup = table.LookupList.LookupTables[lookupIndex];
-            hasChanged |= lookup.TrySubstitution(fontMetrics, table, collection, feature, iterator.Index, count - (iterator.Index - index));
+            _ = lookup.TrySubstitution(fontMetrics, table, collection, feature, iterator.Index, count - (iterator.Index - index));
 
             // Account for substitutions changing the length of the collection.
             if (collection.Count != currentCount)
@@ -72,7 +71,7 @@ public static bool ApplyLookupList(
             }
         }
 
-        return hasChanged;
+        return true;
     }
 
     public static bool ApplyLookupList(
@@ -85,7 +84,6 @@ public static bool ApplyLookupList(
         int index,
         int count)
     {
-        bool hasChanged = false;
         SkippingGlyphIterator iterator = new(fontMetrics, collection, index, lookupFlags);
         foreach (SequenceLookupRecord lookupRecord in records)
         {
@@ -94,10 +92,10 @@ public static bool ApplyLookupList(
             iterator.Index = index;
             iterator.Increment(sequenceIndex);
             LookupTable lookup = table.LookupList.LookupTables[lookupIndex];
-            hasChanged |= lookup.TryUpdatePosition(fontMetrics, table, collection, feature, iterator.Index, count - (iterator.Index - index));
+            _ = lookup.TryUpdatePosition(fontMetrics, table, collection, feature, iterator.Index, count - (iterator.Index - index));
         }
 
-        return hasChanged;
+        return true;
     }
 
     public static bool MatchInputSequence(SkippingGlyphIterator iterator, Tag feature, ushort increment, ushort[] sequence, Span<int> matches)
@@ -222,7 +220,7 @@ public static bool CheckAllCoverages(
         CoverageTable[] lookahead)
     {
         // Check that there are enough context glyphs.
-        if (index - backtrack.Length < 0 || input.Length + lookahead.Length > count)
+        if (index < backtrack.Length || input.Length + lookahead.Length > count)
         {
             return false;
         }
@@ -253,7 +251,8 @@ public static void ApplyAnchor(
         int index,
         AnchorTable baseAnchor,
         MarkRecord markRecord,
-        int baseGlyphIndex)
+        int baseGlyphIndex,
+        Tag feature)
     {
         GlyphShapingData baseData = collection[baseGlyphIndex];
         AnchorXY baseXY = baseAnchor.GetAnchor(fontMetrics, baseData, collection);
@@ -264,18 +263,21 @@ public static void ApplyAnchor(
         markData.Bounds.X = baseXY.XCoordinate - markXY.XCoordinate;
         markData.Bounds.Y = baseXY.YCoordinate - markXY.YCoordinate;
         markData.MarkAttachment = baseGlyphIndex;
+        markData.AppliedFeatures.Add(feature);
     }
 
     public static void ApplyPosition(
         GlyphPositioningCollection collection,
         int index,
-        ValueRecord record)
+        ValueRecord record,
+        Tag feature)
     {
         GlyphShapingData current = collection[index];
         current.Bounds.Width += record.XAdvance;
         current.Bounds.Height += record.YAdvance;
         current.Bounds.X += record.XPlacement;
         current.Bounds.Y += record.YPlacement;
+        current.AppliedFeatures.Add(feature);
     }
 
     public static bool IsMarkGlyph(FontMetrics fontMetrics, ushort glyphId, GlyphShapingData shapingData)

src/SixLabors.Fonts/Tables/AdvancedTypographic/GPos/LookupType1SubTable.cs

@@ -81,7 +81,7 @@ public override bool TryUpdatePosition(
         if (coverage > -1)
         {
             ValueRecord record = this.valueRecord;
-            AdvancedTypographicUtils.ApplyPosition(collection, index, record);
+            AdvancedTypographicUtils.ApplyPosition(collection, index, record, feature);
 
             return true;
         }
@@ -152,7 +152,7 @@ public override bool TryUpdatePosition(
         if (coverage > -1)
         {
             ValueRecord record = this.valueRecords[coverage];
-            AdvancedTypographicUtils.ApplyPosition(collection, index, record);
+            AdvancedTypographicUtils.ApplyPosition(collection, index, record, feature);
 
             return true;
         }

src/SixLabors.Fonts/Tables/AdvancedTypographic/GPos/LookupType2SubTable.cs

@@ -117,10 +117,10 @@ public override bool TryUpdatePosition(
                 if (pairSet.TryGetPairValueRecord(glyphId2, out PairValueRecord pairValueRecord))
                 {
                     ValueRecord record1 = pairValueRecord.ValueRecord1;
-                    AdvancedTypographicUtils.ApplyPosition(collection, index, record1);
+                    AdvancedTypographicUtils.ApplyPosition(collection, index, record1, feature);
 
                     ValueRecord record2 = pairValueRecord.ValueRecord2;
-                    AdvancedTypographicUtils.ApplyPosition(collection, index + 1, record2);
+                    AdvancedTypographicUtils.ApplyPosition(collection, index + 1, record2, feature);
 
                     return true;
                 }
@@ -285,10 +285,10 @@ public override bool TryUpdatePosition(
                 Class2Record class2Record = class1Record.Class2Records[classDef2];
 
                 ValueRecord record1 = class2Record.ValueRecord1;
-                AdvancedTypographicUtils.ApplyPosition(collection, index, record1);
+                AdvancedTypographicUtils.ApplyPosition(collection, index, record1, feature);
 
                 ValueRecord record2 = class2Record.ValueRecord2;
-                AdvancedTypographicUtils.ApplyPosition(collection, index + 1, record2);
+                AdvancedTypographicUtils.ApplyPosition(collection, index + 1, record2, feature);
 
                 return true;
             }

src/SixLabors.Fonts/Tables/AdvancedTypographic/GPos/LookupType4SubTable.cs

@@ -126,7 +126,7 @@ public override bool TryUpdatePosition(
 
             MarkRecord markRecord = this.markArrayTable.MarkRecords[markIndex];
             AnchorTable baseAnchor = this.baseArrayTable.BaseRecords[baseIndex].BaseAnchorTables[markRecord.MarkClass];
-            AdvancedTypographicUtils.ApplyAnchor(fontMetrics, collection, index, baseAnchor, markRecord, baseGlyphIndex);
+            AdvancedTypographicUtils.ApplyAnchor(fontMetrics, collection, index, baseAnchor, markRecord, baseGlyphIndex, feature);
 
             return true;
         }

src/SixLabors.Fonts/Tables/AdvancedTypographic/GPos/LookupType5SubTable.cs

@@ -140,7 +140,7 @@ public override bool TryUpdatePosition(
 
             MarkRecord markRecord = this.markArrayTable.MarkRecords[markIndex];
             AnchorTable baseAnchor = ligatureAttach.ComponentRecords[compIndex].LigatureAnchorTables[markRecord.MarkClass];
-            AdvancedTypographicUtils.ApplyAnchor(fontMetrics, collection, index, baseAnchor, markRecord, baseGlyphIndex);
+            AdvancedTypographicUtils.ApplyAnchor(fontMetrics, collection, index, baseAnchor, markRecord, baseGlyphIndex, feature);
 
             return true;
         }

src/SixLabors.Fonts/Tables/AdvancedTypographic/GPos/LookupType6SubTable.cs

@@ -158,7 +158,7 @@ public override bool TryUpdatePosition(
 
             MarkRecord markRecord = this.mark1ArrayTable.MarkRecords[mark1Index];
             AnchorTable baseAnchor = this.mark2ArrayTable.Mark2Records[mark2Index].MarkAnchorTable[markRecord.MarkClass];
-            AdvancedTypographicUtils.ApplyAnchor(fontMetrics, collection, index, baseAnchor, markRecord, prevIdx);
+            AdvancedTypographicUtils.ApplyAnchor(fontMetrics, collection, index, baseAnchor, markRecord, prevIdx, feature);
 
             return true;
         }

tests/SixLabors.Fonts.Tests/Issues/Issues_451.cs

@@ -0,0 +1,47 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Numerics;
+
+namespace SixLabors.Fonts.Tests.Issues;
+
+public class Issues_451
+{
+    private readonly FontFamily berry = new FontCollection().Add(TestFonts.VeryBerryProRegular);
+
+    [Fact]
+    public void Issue_451_A()
+    {
+        Font font = this.berry.CreateFont(85);
+        TextLayoutTestUtilities.TestLayout(
+            "The",
+            new TextOptions(font)
+            {
+                Origin = new Vector2(0, 50),
+            });
+    }
+
+    [Fact]
+    public void Issue_451_B()
+    {
+        Font font = this.berry.CreateFont(85);
+        TextLayoutTestUtilities.TestLayout(
+            "Th",
+            new TextOptions(font)
+            {
+                Origin = new Vector2(0, 50),
+            });
+    }
+
+    [Fact]
+    public void Issue_451_C()
+    {
+        Font font = this.berry.CreateFont(85);
+        TextLayoutTestUtilities.TestLayout(
+            "The quick brown fox jumps over the lazy dog",
+            new TextOptions(font)
+            {
+                Origin = new Vector2(0, 50),
+            });
+    }
+}

tests/SixLabors.Fonts.Tests/TestFonts.cs

@@ -261,6 +261,8 @@
 
     public static string CharisSILRegular => GetFullPath("CharisSIL-Regular.ttf");
 
+    public static string VeryBerryProRegular => GetFullPath("VeryBerryProRegular.ttf");
+
     public static Stream TwemojiMozillaData() => OpenStream(TwemojiMozillaFile);
 
     public static Stream SegoeuiEmojiData() => OpenStream(SegoeuiEmojiFile);
@@ -312,7 +314,7 @@
 
     private static string GetFullPath(string path)
     {
         string root = Path.GetDirectoryName(new Uri(typeof(TestFonts).GetTypeInfo().Assembly.CodeBase).LocalPath);
 
         string[] paths = new[]
         {