Fix DataGrid IList editing issue (#13845)

* Implement basic writeable IList on DataGridCollectionView hopefully it won't break anything else

* Make preference over DataGridCollectionView in DataGridDataConnection

src/Avalonia.Controls.DataGrid/Collections/DataGridCollectionView.cs

@@ -1153,15 +1153,23 @@ namespace Avalonia.Collections
             get { return GetItemAt(index); }
         }
 
-        bool IList.IsFixedSize => false;
+        bool IList.IsFixedSize => SourceList?.IsFixedSize ?? true;
-        bool IList.IsReadOnly => true;
+        bool IList.IsReadOnly => SourceList?.IsReadOnly ?? true;
         bool ICollection.IsSynchronized => false;
         object ICollection.SyncRoot => this;
 
         object IList.this[int index]
         {
             get => this[index];
-            set => throw new NotSupportedException();
+            set
+            {
+                SourceList[index] = value;
+                if (SourceList is not INotifyCollectionChanged)
+                {
+                    // TODO: implement Replace
+                    ProcessCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset, value));
+                }
+            }
         }
 
         /// <summary>
@@ -3992,9 +4000,36 @@ namespace Avalonia.Collections
             }
         }
 
-        int IList.Add(object value) => throw new NotSupportedException();
+        int IList.Add(object value)
-        void IList.Clear() => throw new NotSupportedException();
+        {
-        void IList.Insert(int index, object value) => throw new NotSupportedException();
+            var index = SourceList.Add(value);
+            if (SourceList is not INotifyCollectionChanged)
+            {
+                ProcessCollectionChanged(
+                    new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, value));
+            }
+            return index;
+        }
+
+        void IList.Clear()
+        {
+            SourceList.Clear();
+            if (SourceList is not INotifyCollectionChanged)
+            {
+                ProcessCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
+            }
+        }
+
+        void IList.Insert(int index, object value) 
+        {
+            SourceList.Insert(index, value);
+            if (SourceList is not INotifyCollectionChanged)
+            {
+                // TODO: implement Insert
+                ProcessCollectionChanged(
+                    new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset, value));
+            }
+        }
         void ICollection.CopyTo(Array array, int index) => InternalList.CopyTo(array, index);
 
         /// <summary>

src/Avalonia.Controls.DataGrid/DataGridDataConnection.cs

@@ -349,15 +349,15 @@ namespace Avalonia.Controls
         {
             Debug.Assert(index >= 0);
 
-            IList list = List;
+            if (DataSource is DataGridCollectionView collectionView)
-            if (list != null)
             {
-                return (index < list.Count) ? list[index] : null;
+                return (index < collectionView.Count) ? collectionView.GetItemAt(index) : null;
             }
 
-            if (DataSource is DataGridCollectionView collectionView)
+            IList list = List;
+            if (list != null)
             {
-                return (index < collectionView.Count) ? collectionView.GetItemAt(index) : null;
+                return (index < list.Count) ? list[index] : null;
             }
 
             IEnumerable enumerable = DataSource;
@@ -419,15 +419,15 @@ namespace Avalonia.Controls
 
         public int IndexOf(object dataItem)
         {
-            IList list = List;
+            if (DataSource is DataGridCollectionView cv)
-            if (list != null)
             {
-                return list.IndexOf(dataItem);
+                return cv.IndexOf(dataItem);
             }
 
-            if (DataSource is DataGridCollectionView cv)
+            IList list = List;
+            if (list != null)
             {
-                return cv.IndexOf(dataItem);
+                return list.IndexOf(dataItem);
             }
 
             IEnumerable enumerable = DataSource;