Cap rendering and editing of extremely long lines

src/AvaloniaEdit.TextMate/TextMateColoringTransformer.cs

@@ -285,6 +285,10 @@ protected override void TransformLine(DocumentLine line, ITextRunConstructionCon
                 if (model == null || document == null || theme == null || brushes == null)
                     return;
 
+                // for long lines, do not tokenize and colorize for performance reasons
+                if (line.Length > VisualLine.LENGTH_LIMIT)
+                    return;
+
                 int lineNumber = line.LineNumber;
 
                 var tokens = model.GetLineTokens(lineNumber - 1);

src/AvaloniaEdit/Editing/EditingCommandHandler.cs

@@ -248,7 +248,7 @@ public static void OnTab(object target, RoutedEventArgs args)
                         while (true)
                         {
                             var offset = current.Offset;
-                            if (textArea.ReadOnlySectionProvider.CanInsert(offset))
+                            if (textArea.CanInsert(offset))
                                 textArea.Document.Replace(offset, 0, textArea.Options.IndentationString,
                                     OffsetChangeMappingType.KeepAnchorBeforeInsertion);
                             if (current == end)
@@ -499,7 +499,7 @@ private static void CanPaste(object target, CanExecuteRoutedEventArgs args)
             var textArea = GetTextArea(target);
             if (textArea is { Document: not null })
             {
-                args.CanExecute = textArea.ReadOnlySectionProvider.CanInsert(textArea.Caret.Offset);
+                args.CanExecute = textArea.CanInsert(textArea.Caret.Offset);
                 args.Handled = true;
             }
         }

src/AvaloniaEdit/Editing/EmptySelection.cs

@@ -68,7 +68,7 @@ public override void ReplaceSelectionWithText(string newText)
             newText = AddSpacesIfRequired(newText, TextArea.Caret.Position, TextArea.Caret.Position);
             if (newText.Length > 0)
             {
-                if (TextArea.ReadOnlySectionProvider.CanInsert(TextArea.Caret.Offset))
+                if (TextArea.CanInsert(TextArea.Caret.Offset))
                 {
                     TextArea.Document.Insert(TextArea.Caret.Offset, newText);
                 }

src/AvaloniaEdit/Editing/LongLineEditProtection.cs

@@ -0,0 +1,74 @@
+using System;
+using System.Collections.Generic;
+
+using AvaloniaEdit.Document;
+using AvaloniaEdit.Rendering;
+
+namespace AvaloniaEdit.Editing
+{
+    /// <summary>
+    /// Disallow edits in the collapsed tail of long lines (see <see cref="VisualLine.LENGTH_LIMIT"/>):
+    /// </summary>
+    internal static class LongLineEditProtection
+    {
+        internal static bool CanInsert(TextDocument document, int offset)
+        {
+            if (document == null)
+                return true;
+
+            DocumentLine line = document.GetLineByOffset(offset);
+            return offset - line.Offset < VisualLine.LENGTH_LIMIT;
+        }
+
+        internal static IEnumerable<ISegment> GetDeletableSegments(
+            TextDocument document, ISegment segment)
+        {
+            if (segment == null)
+                throw new ArgumentNullException(nameof(segment));
+
+            if (document == null)
+            {
+                yield return segment;
+                yield break;
+            }
+
+            if (segment.Length == 0)
+            {
+                if (CanInsert(document, segment.Offset))
+                    yield return segment;
+                yield break;
+            }
+
+            int deletableStart = segment.Offset;
+            int segmentEnd = segment.EndOffset;
+
+            DocumentLine line = document.GetLineByOffset(segment.Offset);
+            while (line != null && line.Offset < segmentEnd)
+            {
+                if (line.Length > VisualLine.LENGTH_LIMIT)
+                {
+                    // protect the hidden text plus the line delimiter; deleting the
+                    // delimiter would merge the next line into the hidden region
+                    int protectedStart = line.Offset + VisualLine.LENGTH_LIMIT;
+                    int protectedEnd = line.Offset + line.TotalLength;
+
+                    if (protectedStart < segmentEnd && protectedEnd > deletableStart)
+                    {
+                        if (protectedStart > deletableStart)
+                        {
+                            yield return new SimpleSegment(
+                                deletableStart, protectedStart - deletableStart);
+                        }
+
+                        deletableStart = Math.Min(protectedEnd, segmentEnd);
+                    }
+                }
+
+                line = line.NextLine;
+            }
+
+            if (deletableStart < segmentEnd)
+                yield return new SimpleSegment(deletableStart, segmentEnd - deletableStart);
+        }
+    }
+}

src/AvaloniaEdit/Editing/RectangleSelection.cs

@@ -328,7 +328,7 @@ private void ReplaceSingleLineText(TextArea textArea, SelectionSegment lineSegme
         {
             if (lineSegment.Length == 0)
             {
-                if (newText.Length > 0 && textArea.ReadOnlySectionProvider.CanInsert(lineSegment.StartOffset))
+                if (newText.Length > 0 && textArea.CanInsert(lineSegment.StartOffset))
                 {
                     newText = AddSpacesIfRequired(newText, new TextViewPosition(_document.GetLocation(lineSegment.StartOffset), lineSegment.StartVisualColumn), new TextViewPosition(_document.GetLocation(lineSegment.EndOffset), lineSegment.EndVisualColumn));
                     textArea.Document.Insert(lineSegment.StartOffset, newText);

src/AvaloniaEdit/Editing/SelectionMouseHandler.cs

@@ -188,7 +188,7 @@ DragDropEffects GetEffect(DragEventArgs e)
                 {
                     TextArea.Caret.Position = new TextViewPosition(TextArea.Document.GetLocation(offset), visualColumn) { IsAtEndOfLine = isAtEndOfLine };
                     TextArea.Caret.DesiredXPos = double.NaN;
-                    if (TextArea.ReadOnlySectionProvider.CanInsert(offset))
+                    if (TextArea.CanInsert(offset))
                     {
                         if ((e.DragEffects & DragDropEffects.Move) == DragDropEffects.Move
                             && (e.KeyModifiers & KeyModifiers.Control) != KeyModifiers.Control)

src/AvaloniaEdit/Editing/TextArea.cs

@@ -925,7 +925,7 @@ internal void RemoveSelectedText()
             {
                 foreach (var s in _selection.Segments)
                 {
-                    Debug.Assert(!ReadOnlySectionProvider.GetDeletableSegments(s).Any());
+                    Debug.Assert(GetDeletableSegments(s).Length == 0);
                 }
             }
 #endif
@@ -940,12 +940,20 @@ internal void ReplaceSelectionWithText(string newText)
             _selection.ReplaceSelectionWithText(newText);
         }
 
+        internal bool CanInsert(int offset)
+        {
+            return ReadOnlySectionProvider.CanInsert(offset) &&
+                   LongLineEditProtection.CanInsert(Document, offset);
+        }
+
         internal ISegment[] GetDeletableSegments(ISegment segment)
         {
             var deletableSegments = ReadOnlySectionProvider.GetDeletableSegments(segment);
             if (deletableSegments == null)
                 throw new InvalidOperationException("ReadOnlySectionProvider.GetDeletableSegments returned null");
-            var array = deletableSegments.ToArray();
+            var array = deletableSegments
+                .SelectMany(s => LongLineEditProtection.GetDeletableSegments(Document, s))
+                .ToArray();
             var lastIndex = segment.Offset;
             foreach (var t in array)
             {

src/AvaloniaEdit/Rendering/VisualLine.cs

@@ -41,6 +41,7 @@ namespace AvaloniaEdit.Rendering
     /// </summary>
     public sealed class VisualLine
     {
+        public const string LENGTH_LIMIT_MARKER = "...";
         public const int LENGTH_LIMIT = 3000;
 
         private enum LifetimePhase : byte
@@ -172,7 +173,10 @@ void PerformVisualElementConstruction(IReadOnlyList<VisualLineElementGenerator>
             // for long lines, do not run the element generators for performance reasons
             if (lineLength > LENGTH_LIMIT)
             {
-                _elements.Add(new VisualLineText(this, lineLength));
+                // shape only the first LENGTH_LIMIT characters and collapse the remainder
+                // into a marker. Otherwise stalls the UI thread.
+                _elements.Add(new VisualLineText(this, LENGTH_LIMIT));
+                _elements.Add(new FormattedTextElement(LENGTH_LIMIT_MARKER, lineLength - LENGTH_LIMIT));
                 return;
             }