Add TextBox UndoLimit

src/Avalonia.Controls/TextBox.cs

@@ -138,6 +138,13 @@ namespace Avalonia.Controls
                 nameof(IsUndoEnabled), 
                 defaultValue: true);
 
+        public static readonly DirectProperty<TextBox, int> UndoLimitProperty =
+            AvaloniaProperty.RegisterDirect<TextBox, int>(
+                nameof(UndoLimit),
+                o => o._undoRedoHelper.Limit,
+                (o, v) => o._undoRedoHelper.Limit = v,
+                unsetValue: -1);
+
         struct UndoRedoState : IEquatable<UndoRedoState>
         {
             public string Text { get; }
@@ -472,6 +479,19 @@ namespace Avalonia.Controls
             }
         }
 
+        public int UndoLimit
+        {
+            get { return GetValue(UndoLimitProperty); }
+            set
+            {
+                SetValue(UndoLimitProperty, value);
+                // from docs at
+                // https://docs.microsoft.com/en-us/dotnet/api/system.windows.controls.primitives.textboxbase.isundoenabled:
+                // "Setting UndoLimit clears the undo queue."
+                _undoRedoHelper.Clear();
+            }
+        }
+
         protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
         {
             _presenter = e.NameScope.Get<TextPresenter>("PART_TextPresenter");

src/Avalonia.Controls/Utils/UndoRedoHelper.cs

@@ -22,6 +22,10 @@ namespace Avalonia.Controls.Utils
 
         private LinkedListNode<TState> _currentNode;
 
+        /// <summary>
+        /// Maximum number of states this helper can store for undo/redo.
+        /// If -1, no limit is imposed.
+        /// </summary>
         public int Limit { get; set; } = 10;
 
         public UndoRedoHelper(IUndoRedoHost host)
@@ -54,7 +58,10 @@ namespace Avalonia.Controls.Utils
         public bool HasState => _currentNode != null;
         public void UpdateLastState(TState state)
         {
-            _states.Last.Value = state;
+            if (_states.Last != null)
+            {
+                _states.Last.Value = state;
+            } 
         }
 
         public void UpdateLastState()
@@ -86,7 +93,7 @@ namespace Avalonia.Controls.Utils
                     DiscardRedo();
                 _states.AddLast(current);
                 _currentNode = _states.Last;
-                if (_states.Count > Limit)
+                if (Limit != -1 && _states.Count > Limit)
                     _states.RemoveFirst();
             }
         }