When replacing the whole document, ensure the deleted lines have IsDeleted set to true.

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit.Tests/Document/LineManagerTests.cs

@@ -18,6 +18,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using NUnit.Framework;
 
 namespace ICSharpCode.AvalonEdit.Document
@@ -46,10 +47,16 @@ namespace ICSharpCode.AvalonEdit.Document
 		{
 			document.Text = "Hello,\nWorld!";
 			Assert.AreEqual(2, document.LineCount);
+			var oldLines = document.Lines.ToArray();
 			document.Text = "";
 			Assert.AreEqual("", document.Text);
 			Assert.AreEqual(0, document.TextLength);
 			Assert.AreEqual(1, document.LineCount);
+			Assert.AreSame(oldLines[0], document.Lines.Single());
+			Assert.IsFalse(oldLines[0].IsDeleted);
+			Assert.IsTrue(oldLines[1].IsDeleted);
+			Assert.IsNull(oldLines[0].NextLine);
+			Assert.IsNull(oldLines[1].PreviousLine);
 		}
 		
 		[Test]

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/Document/LineManager.cs

@@ -105,6 +105,13 @@ namespace ICSharpCode.AvalonEdit.Document
 		{
 			// keep the first document line
 			DocumentLine ls = documentLineTree.GetByNumber(1);
+			// but mark all other lines as deleted, and detach them from the other nodes
+			for (DocumentLine line = ls.NextLine; line != null; line = line.NextLine) {
+				line.isDeleted = true;
+				line.parent = line.left = line.right = null;
+			}
+			// Reset the first line to detach it from the deleted lines
+			ls.ResetLine();
 			SimpleSegment ds = NewLineFinder.NextNewLine(document, 0);
 			List<DocumentLine> lines = new List<DocumentLine>();
 			int lastDelimiterEnd = 0;
@@ -117,7 +124,6 @@ namespace ICSharpCode.AvalonEdit.Document
 				ls = new DocumentLine(document);
 				ds = NewLineFinder.NextNewLine(document, lastDelimiterEnd);
 			}
-			ls.ResetLine();
 			ls.TotalLength = document.TextLength - lastDelimiterEnd;
 			lines.Add(ls);
 			documentLineTree.RebuildTree(lines);