Trim invalid selection when Source first assigned.

`SelectionModel` can have a selection set before its `Source`  is initialized. In this case, be sure to trim invalid selections from the model before continuing.

Fixes #3919

src/Avalonia.Controls/IndexRange.cs

@@ -132,6 +132,53 @@ namespace Avalonia.Controls
             return result;
         }
 
+        public static int Intersect(
+            IList<IndexRange> ranges,
+            IndexRange range,
+            IList<IndexRange>? removed = null)
+        {
+            var result = 0;
+
+            for (var i = 0; i < ranges.Count && range != s_invalid; ++i)
+            {
+                var existing = ranges[i];
+
+                if (existing.End < range.Begin || existing.Begin > range.End)
+                {
+                    removed?.Add(existing);
+                    ranges.RemoveAt(i--);
+                    result += existing.Count;
+                }
+                else
+                {
+                    if (existing.Begin < range.Begin)
+                    {
+                        var except = new IndexRange(existing.Begin, range.Begin - 1);
+                        removed?.Add(except);
+                        ranges[i] = existing = new IndexRange(range.Begin, existing.End);
+                        result += except.Count;
+                    }
+
+                    if (existing.End > range.End)
+                    {
+                        var except = new IndexRange(range.End + 1, existing.End);
+                        removed?.Add(except);
+                        ranges[i] = new IndexRange(existing.Begin, range.End);
+                        result += except.Count;
+                    }
+                }
+            }
+
+            MergeRanges(ranges);
+
+            if (removed is object)
+            {
+                MergeRanges(removed);
+            }
+
+            return result;
+        }
+
         public static int Remove(
             IList<IndexRange> ranges,
             IndexRange range,

src/Avalonia.Controls/SelectionModel.cs

@@ -45,12 +45,9 @@ namespace Avalonia.Controls
 
                     if (_rootNode.Source != null)
                     {
-                        if (_rootNode.Source != null)
+                        using (var operation = new Operation(this))
                         {
-                            using (var operation = new Operation(this))
-                            {
-                                ClearSelection(resetAnchor: true);
-                            }
+                            ClearSelection(resetAnchor: true);
                         }
                     }
 

src/Avalonia.Controls/SelectionModelChangeSet.cs

@@ -135,7 +135,7 @@ namespace Avalonia.Controls
                 if (index >= currentIndex && index < currentIndex + currentCount)
                 {
                     int targetIndex = GetIndexAt(getRanges(info), index - currentIndex);
-                    item = info.Items?.GetAt(targetIndex);
+                    item = info.Items?.Count > targetIndex ? info.Items?.GetAt(targetIndex) : null;
                     break;
                 }
 

src/Avalonia.Controls/SelectionNode.cs

@@ -101,6 +101,7 @@ namespace Avalonia.Controls
 
                     ItemsSourceView = newDataSource;
 
+                    TrimInvalidSelections();
                     PopulateSelectedItemsFromSelectedIndices();
                     HookupCollectionChangedHandler();
                     OnSelectionChanged();
@@ -108,6 +109,26 @@ namespace Avalonia.Controls
             }
         }
 
+        private void TrimInvalidSelections()
+        {
+            if (_selected == null || ItemsSourceView == null)
+            {
+                return;
+            }
+
+            var validRange = ItemsSourceView.Count > 0 ? new IndexRange(0, ItemsSourceView.Count - 1) : new IndexRange(-1, -1);
+            var removed = new List<IndexRange>();
+            var removedCount = IndexRange.Intersect(_selected, validRange, removed);
+
+            if (removedCount > 0)
+            {
+                using var operation = _manager.Update();
+                SelectedCount -= removedCount;
+                OnSelectionChanged();
+                _operation!.Deselected(removed);
+            }
+        }
+
         public ItemsSourceView? ItemsSourceView { get; private set; }
         public int DataCount => ItemsSourceView?.Count ?? 0;
         public int ChildrenNodeCount => _childrenNodes.Count;

tests/Avalonia.Controls.UnitTests/IndexRangeTests.cs

@@ -127,6 +127,88 @@ namespace Avalonia.Controls.UnitTests
             Assert.Empty(selected);
         }
 
+        [Fact]
+        public void Intersect_Should_Remove_Items_From_Beginning()
+        {
+            var ranges = new List<IndexRange> { new IndexRange(0, 10) };
+            var removed = new List<IndexRange>();
+            var result = IndexRange.Intersect(ranges, new IndexRange(2, 12), removed);
+
+            Assert.Equal(2, result);
+            Assert.Equal(new[] { new IndexRange(2, 10) }, ranges);
+            Assert.Equal(new[] { new IndexRange(0, 1) }, removed);
+        }
+
+        [Fact]
+        public void Intersect_Should_Remove_Items_From_End()
+        {
+            var ranges = new List<IndexRange> { new IndexRange(0, 10) };
+            var removed = new List<IndexRange>();
+            var result = IndexRange.Intersect(ranges, new IndexRange(0, 8), removed);
+
+            Assert.Equal(2, result);
+            Assert.Equal(new[] { new IndexRange(0, 8) }, ranges);
+            Assert.Equal(new[] { new IndexRange(9, 10) }, removed);
+        }
+
+        [Fact]
+        public void Intersect_Should_Remove_Entire_Range_Start()
+        {
+            var ranges = new List<IndexRange> { new IndexRange(0, 5), new IndexRange(6, 10) };
+            var removed = new List<IndexRange>();
+            var result = IndexRange.Intersect(ranges, new IndexRange(6, 10), removed);
+
+            Assert.Equal(6, result);
+            Assert.Equal(new[] { new IndexRange(6, 10) }, ranges);
+            Assert.Equal(new[] { new IndexRange(0, 5) }, removed);
+        }
+
+        [Fact]
+        public void Intersect_Should_Remove_Entire_Range_End()
+        {
+            var ranges = new List<IndexRange> { new IndexRange(0, 5), new IndexRange(6, 10) };
+            var removed = new List<IndexRange>();
+            var result = IndexRange.Intersect(ranges, new IndexRange(0, 4), removed);
+
+            Assert.Equal(6, result);
+            Assert.Equal(new[] { new IndexRange(0, 4) }, ranges);
+            Assert.Equal(new[] { new IndexRange(5, 10) }, removed);
+        }
+
+        [Fact]
+        public void Intersect_Should_Remove_Entire_Range_Start_End()
+        {
+            var ranges = new List<IndexRange> 
+            { 
+                new IndexRange(0, 2),
+                new IndexRange(3, 7), 
+                new IndexRange(8, 10) 
+            };
+            var removed = new List<IndexRange>();
+            var result = IndexRange.Intersect(ranges, new IndexRange(3, 7), removed);
+
+            Assert.Equal(6, result);
+            Assert.Equal(new[] { new IndexRange(3, 7) }, ranges);
+            Assert.Equal(new[] { new IndexRange(0, 2), new IndexRange(8, 10) }, removed);
+        }
+
+        [Fact]
+        public void Intersect_Should_Remove_Entire_And_Partial_Range_Start_End()
+        {
+            var ranges = new List<IndexRange>
+            {
+                new IndexRange(0, 2),
+                new IndexRange(3, 7),
+                new IndexRange(8, 10)
+            };
+            var removed = new List<IndexRange>();
+            var result = IndexRange.Intersect(ranges, new IndexRange(4, 6), removed);
+
+            Assert.Equal(8, result);
+            Assert.Equal(new[] { new IndexRange(4, 6) }, ranges);
+            Assert.Equal(new[] { new IndexRange(0, 3), new IndexRange(7, 10) }, removed);
+        }
+
         [Fact]
         public void Remove_Should_Remove_Entire_Range()
         {

tests/Avalonia.Controls.UnitTests/SelectionModelTests.cs

@@ -1775,7 +1775,17 @@ namespace Avalonia.Controls.UnitTests
         }
 
         [Fact]
-        public void Assigning_Source_With_Less_Items_Than_Selection_Trims_Selection()
+        public void Initializing_Source_With_Less_Items_Than_Selection_Trims_Selection()
+        {
+            var data = new[] { "foo", "bar", "baz" };
+            var target = new SelectionModel();
+            target.SelectedIndex = new IndexPath(4);
+            target.Source = data;
+            Assert.Empty(target.SelectedIndices);
+        }
+
+        [Fact]
+        public void Initializing_Source_With_Less_Items_Than_Selection_Trims_Selection_RetainSelection()
         {
             var data = new[] { "foo", "bar", "baz" };
             var target = new SelectionModel { RetainSelectionOnReset = true };
@@ -1785,7 +1795,7 @@ namespace Avalonia.Controls.UnitTests
         }
 
         [Fact]
-        public void Assigning_Source_With_Less_Items_Than_Multiple_Selection_Trims_Selection()
+        public void Initializing_Source_With_Less_Items_Than_Multiple_Selection_Trims_Selection()
         {
             var data = new[] { "foo", "bar", "baz" };
             var target = new SelectionModel { RetainSelectionOnReset = true };
@@ -1796,17 +1806,31 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(new IndexPath(2), target.SelectedIndices.First());
         }
 
-        // test that going from non-null source to non-null source clears selection
         [Fact]
-        public void Changing_Source_With_Valid_IndexPath_Retains_Selection()
+        public void Initializing_Source_With_Less_Items_Than_Selection_Raises_SelectionChanged()
         {
-            var data = new[] { "foo", "bar", "baz", "boo", "hoo" };
-            var smallerData = new[] { "foo", "bar", "baz" };
-            var target = new SelectionModel { RetainSelectionOnReset = true };
+            var data = new[] { "foo", "bar", "baz" };
+            var target = new SelectionModel();
+            var raised = 0;
+
+            target.SelectedIndex = new IndexPath(4);
+
+            target.SelectionChanged += (s, e) =>
+            {
+                if (raised == 0)
+                {
+                    Assert.Equal(new[] { Path(4) }, e.DeselectedIndices);
+                    Assert.Equal(new object[] { null }, e.DeselectedItems);
+                    Assert.Empty(e.SelectedIndices);
+                    Assert.Empty(e.SelectedItems);
+                }
+
+                ++raised;
+            };
+
             target.Source = data;
-            target.SelectedIndex = new IndexPath(2);
-            target.Source = smallerData;
-            Assert.Equal(1, target.SelectedIndices.Count);
+            
+            Assert.Equal(2, raised);
         }
 
         private int GetSubscriberCount(AvaloniaList<object> list)