Character-only mutations should not overwrite paragraph-bound attributes in paragraph following mutation

Sources/FoundationEssentials/AttributedString/AttributedString+CharacterView.swift

@@ -283,7 +283,7 @@ extension AttributedString.CharacterView: RangeReplaceableCollection {
         _guts.runs.replaceUTF8Subrange(utf8Range, with: CollectionOfOne(run))
 
         // Invalidate attributes surrounding the affected range. (Phase 2)
-        _guts._finalizeStringMutation(state)
+        _guts._finalizeStringMutation(state, type: .characters)
     }
 
     public mutating func replaceSubrange(

Sources/FoundationEssentials/AttributedString/AttributedString+Guts.swift

@@ -457,15 +457,16 @@ extension AttributedString.Guts {
     }
 
     func _finalizeStringMutation(
-        _ state: (mutationStartUTF8Offset: Int, isInsertion: Bool, oldUTF8Count: Int, invalidationRange: Range<Int>)
+        _ state: (mutationStartUTF8Offset: Int, isInsertion: Bool, oldUTF8Count: Int, invalidationRange: Range<Int>),
+        type: _MutationType
     ) {
         let utf8Delta = self.string.utf8.count - state.oldUTF8Count
         self._finalizeTrackedIndicesUpdate(mutationStartOffset: state.mutationStartUTF8Offset, isInsertion: state.isInsertion, utf8LengthDelta: utf8Delta)
         let lower = state.invalidationRange.lowerBound
         let upper = state.invalidationRange.upperBound + utf8Delta
         self.enforceAttributeConstraintsAfterMutation(
             in: lower ..< upper,
-            type: .attributesAndCharacters)
+            type: type)
     }
 
     func replaceSubrange(
@@ -493,7 +494,7 @@ extension AttributedString.Guts {
             let state = _prepareStringMutation(in: range)
             self.string.unicodeScalars.replaceSubrange(range, with: replacementScalars)
             self.runs.replaceUTF8Subrange(utf8TargetRange, with: replacementRuns)
-            _finalizeStringMutation(state)
+            _finalizeStringMutation(state, type: .attributesAndCharacters)
         } else {
             self.runs.replaceUTF8Subrange(utf8TargetRange, with: replacementRuns)
             self.enforceAttributeConstraintsAfterMutation(in: range._utf8OffsetRange, type: .attributes)

Sources/FoundationEssentials/AttributedString/AttributedString+UnicodeScalarView.swift

@@ -264,7 +264,7 @@ extension AttributedString.UnicodeScalarView: RangeReplaceableCollection {
         _guts.runs.replaceUTF8Subrange(utf8Range, with: CollectionOfOne(run))
 
         // Invalidate attributes surrounding the affected range. (Phase 2)
-        _guts._finalizeStringMutation(state)
+        _guts._finalizeStringMutation(state, type: .characters)
     }
 
     public mutating func replaceSubrange(

Sources/FoundationEssentials/AttributedString/AttributedStringAttributeConstrainingBehavior.swift

@@ -172,6 +172,7 @@ extension AttributedString.Guts {
     enum _MutationType {
         case attributes
         case attributesAndCharacters
+        case characters
     }
 
     /// Removes full runs of any attributes that have declared an
@@ -291,7 +292,7 @@ extension AttributedString.Guts {
             ).updateEach { attributes, _, modified in
                 modified = attributes.matchStyle(of: paragraphStyle, for: .paragraph)
             }
-        } else if type == .attributesAndCharacters {
+        } else {
             // If any character mutations took place, we apply the constrained styles from the start of each paragraph to the remainder of the paragraph
             // The mutation range itself is already fixed-up, so we just need to correct the starting and ending paragraphs
 
@@ -324,8 +325,18 @@ extension AttributedString.Guts {
                     (startParagraph?.upperBound ?? 0) < utf8Range.upperBound,
                     _needsParagraphFixing(from: utf8Range.upperBound - 1, to: utf8Range.upperBound)
                 {
-                    let r = _paragraphExtending(from: string.index(before: strRange.upperBound))
-                    endParagraph = r._utf8OffsetRange
+                    let justInsideIndex = string.index(before: strRange.upperBound)
+                    let r = if type == .characters {
+                        // If a character-only mutation, since we apply to the start of the paragraph we need the full real paragraph range
+                        _paragraph(in: Range(uncheckedBounds: (justInsideIndex, justInsideIndex)))
+                    } else {
+                        // If an attribute + character mutation, we don't need the real start of the paragraph
+                        _paragraphExtending(from: justInsideIndex)
+                    }
+                    // If the ending paragraph starts before the mutation we shouldn't store it here as it will incorrectly overwrite the expansion from startParagraph
+                    if r.lowerBound >= strRange.lowerBound {
+                        endParagraph = r._utf8OffsetRange
+                    }
                 }
             }
 
@@ -336,12 +347,21 @@ extension AttributedString.Guts {
                     from: startParagraph.lowerBound,
                     to: utf8Range.lowerBound ..< startParagraph.upperBound)
             }
-            // If the end paragraph extends beyond the mutation, fixup the range outside the mutation
+            // If the end paragraph extends beyond the mutation, fixup the paragraph
             if let endParagraph, endParagraph.upperBound > utf8Range.upperBound {
-                _applyStyle(
-                    type: .paragraph,
-                    from: endParagraph.lowerBound,
-                    to: utf8Range.upperBound ..< endParagraph.upperBound)
+                if type == .attributesAndCharacters {
+                    // If an attribute + characters mutation, the newly applied attributes expand to cover the remainder of the paragraph
+                    _applyStyle(
+                        type: .paragraph,
+                        from: endParagraph.lowerBound,
+                        to: utf8Range.upperBound ..< endParagraph.upperBound)
+                } else {
+                    // If a character-only mutation, the attributes from the remainder of the paragraph expand to cover the newly inserted text
+                    _applyStyle(
+                        type: .paragraph,
+                        from: utf8Range.upperBound,
+                        to: endParagraph.lowerBound ..< utf8Range.upperBound)
+                }
             }
         }
     }

Tests/FoundationEssentialsTests/AttributedString/AttributedStringConstrainingBehaviorTests.swift

@@ -223,6 +223,10 @@ class TestAttributedStringConstrainingBehavior: XCTestCase {
         result.characters.insert(contentsOf: "Test", at: result.index(result.startIndex, offsetByCharacters: 2))
         verify(string: result, matches: [("HeTestllo, world\n", 1), ("Next Paragraph", 2)], for: \.testParagraphConstrained)
 
+        result = str
+        result.characters.insert(contentsOf: "Test", at: result.index(result.startIndex, offsetByCharacters: 13))
+        verify(string: result, matches: [("Hello, world\n", 1), ("TestNext Paragraph", 2)], for: \.testParagraphConstrained)
+
         result = str
         result.characters.append(contentsOf: "Test")
         verify(string: result, matches: [("Hello, world\n", 1), ("Next ParagraphTest", 2)], for: \.testParagraphConstrained)
@@ -257,7 +261,7 @@ class TestAttributedStringConstrainingBehavior: XCTestCase {
 
         result = str
         result.characters.replaceSubrange(result.index(result.startIndex, offsetByCharacters: 8) ..< result.index(result.startIndex, offsetByCharacters: 15), with: "Test\nReplacement")
-        verify(string: result, matches: [("Hello, wTest\n", 1), ("Replacementxt Paragraph", 1)], for: \.testParagraphConstrained)
+        verify(string: result, matches: [("Hello, wTest\n", 1), ("Replacementxt Paragraph", 2)], for: \.testParagraphConstrained)
     }
 
     func testParagraphAttributedTextMutation() {