diff --git a/src/OneWare.Essentials/EditorExtensions/TextModificationService.cs b/src/OneWare.Essentials/EditorExtensions/TextModificationService.cs
index 28f60c1d..3765e6ca 100644
--- a/src/OneWare.Essentials/EditorExtensions/TextModificationService.cs
+++ b/src/OneWare.Essentials/EditorExtensions/TextModificationService.cs
@@ -45,12 +45,19 @@ public void SetModification(string key, params TextModificationSegment[] segment
                 if (s.StartOffset >= f.StartOffset)
                 {
                     if (s.StartOffset <= f.EndOffset) continue; //Completely overlapped
-                    if (s.EndOffset > f.EndOffset + 1) s.StartOffset = f.EndOffset + 1;
+                    if (s.EndOffset > f.EndOffset + 1)
+                    {
+                        var endOffset = s.EndOffset;
+                        s.StartOffset = f.EndOffset + 1;
+                        s.EndOffset = endOffset;
+                    }
                 }
 
                 if (s.StartOffset < f.StartOffset)
                     if (s.EndOffset > f.EndOffset - 1)
                         s.EndOffset = f.EndOffset - 1;
+
+                if (s.Length <= 0) continue;
             }
 
             m.Add(s);
@@ -67,13 +74,21 @@ protected override void ColorizeLine(DocumentLine line)
 
                 foreach (var overlap in overlaps)
                 {
-                    if (overlap.EndOffset > line.EndOffset) overlap.EndOffset = line.EndOffset;
-                    if (overlap.StartOffset < line.Offset) overlap.StartOffset = line.Offset;
-                    ChangeLinePart(overlap.StartOffset, overlap.EndOffset, x => ApplyChanges(x, overlap));
+                    if (!TryClampToLine(overlap, line, out var startOffset, out var endOffset)) continue;
+                    ChangeLinePart(startOffset, endOffset, x => ApplyChanges(x, overlap));
                 }
             }
     }
 
+    // Must not mutate the segment: it is shared by all lines it spans, and setting
+    // TextSegment.StartOffset moves the segment (keeps its length) instead of trimming it.
+    internal static bool TryClampToLine(ISegment segment, DocumentLine line, out int startOffset, out int endOffset)
+    {
+        startOffset = Math.Max(segment.Offset, line.Offset);
+        endOffset = Math.Min(segment.EndOffset, line.EndOffset);
+        return endOffset > startOffset;
+    }
+
     private void ApplyChanges(VisualLineElement element, TextModificationSegment segment)
     {
         if (segment.Foreground != null) element.TextRunProperties.SetForegroundBrush(segment.Foreground);
diff --git a/src/OneWare.Essentials/OneWare.Essentials.csproj b/src/OneWare.Essentials/OneWare.Essentials.csproj
index e9f74ef0..78efcdc2 100644
--- a/src/OneWare.Essentials/OneWare.Essentials.csproj
+++ b/src/OneWare.Essentials/OneWare.Essentials.csproj
@@ -40,4 +40,8 @@
         <None Include="readme.md" Pack="true" PackagePath=""/>
     </ItemGroup>
 
+    <ItemGroup>
+        <InternalsVisibleTo Include="OneWare.Essentials.UnitTests"/>
+    </ItemGroup>
+
 </Project> 
diff --git a/tests/OneWare.Essentials.UnitTests/TextModificationServiceTests.cs b/tests/OneWare.Essentials.UnitTests/TextModificationServiceTests.cs
new file mode 100644
index 00000000..3a6b2f94
--- /dev/null
+++ b/tests/OneWare.Essentials.UnitTests/TextModificationServiceTests.cs
@@ -0,0 +1,38 @@
+using AvaloniaEdit.Document;
+using OneWare.Essentials.EditorExtensions;
+using Xunit;
+
+namespace OneWare.Essentials.UnitTests;
+
+public class TextModificationServiceTests
+{
+    [Fact]
+    public void TryClampToLine_MultiLineSegment_StaysWithinEachLineAndDoesNotMutateSegment()
+    {
+        var document = new TextDocument { Text = "first\nsecond line\nthird" };
+        var segment = new TextModificationSegment(2, document.TextLength - 2);
+        var originalStart = segment.StartOffset;
+        var originalEnd = segment.EndOffset;
+
+        foreach (var line in document.Lines)
+        {
+            Assert.True(TextModificationService.TryClampToLine(segment, line, out var start, out var end));
+            Assert.InRange(start, line.Offset, line.EndOffset);
+            Assert.InRange(end, line.Offset, line.EndOffset);
+            Assert.True(end > start);
+        }
+
+        Assert.Equal(originalStart, segment.StartOffset);
+        Assert.Equal(originalEnd, segment.EndOffset);
+    }
+
+    [Fact]
+    public void TryClampToLine_SegmentOnlyTouchingLineDelimiter_ReturnsFalse()
+    {
+        var document = new TextDocument { Text = "ab\ncd" };
+        var firstLine = document.GetLineByNumber(1);
+        var segment = new TextModificationSegment(firstLine.EndOffset, firstLine.EndOffset + 1);
+
+        Assert.False(TextModificationService.TryClampToLine(segment, firstLine, out _, out _));
+    }
+}