Prevent AutoCompleteBox getting stuck in a state where it can't drop down. (#17074)

* Added failing AutoCompleteBox test.

* Make sure that flags are reset if exits early.

Wrap everything in a `try`...`finally` block so that they get reset even on exception.

src/Avalonia.Controls/AutoCompleteBox/AutoCompleteBox.cs

@@ -1403,74 +1403,75 @@ namespace Avalonia.Controls
             // Indicate that filtering is ongoing
             _filterInAction = true;
 
-            if (_items == null)
+            try
             {
-                ClearView();
-                return;
-            }
+                if (_items == null)
+                {
+                    ClearView();
+                    return;
+                }
 
-            // Cache the current text value
-            string text = Text ?? string.Empty;
+                // Cache the current text value
+                string text = Text ?? string.Empty;
 
-            // Determine if any filtering mode is on
-            bool stringFiltering = TextFilter != null;
-            bool objectFiltering = FilterMode == AutoCompleteFilterMode.Custom && TextFilter == null;
-            
-            List<object> items = _items;
+                // Determine if any filtering mode is on
+                bool stringFiltering = TextFilter != null;
+                bool objectFiltering = FilterMode == AutoCompleteFilterMode.Custom && TextFilter == null;
 
-            // cache properties
-            var textFilter = TextFilter;
-            var itemFilter = ItemFilter;
-            var _newViewItems = new Collection<object>();
-            
-            // if the mode is objectFiltering and itemFilter is null, we throw an exception
-            if (objectFiltering && itemFilter is null)
-            {
-                // indicate that filtering is not ongoing anymore
-                _filterInAction = false;
-                _cancelRequested = false;
-                
-                throw new Exception(
-                    "ItemFilter property can not be null when FilterMode has value AutoCompleteFilterMode.Custom");
-            }
+                List<object> items = _items;
 
-            foreach (object item in items)
-            {
-                // Exit the fitter when requested if cancellation is requested
-                if (_cancelRequested)
+                // cache properties
+                var textFilter = TextFilter;
+                var itemFilter = ItemFilter;
+                var _newViewItems = new Collection<object>();
+
+                // if the mode is objectFiltering and itemFilter is null, we throw an exception
+                if (objectFiltering && itemFilter is null)
                 {
-                    return;
+                    throw new Exception(
+                        "ItemFilter property can not be null when FilterMode has value AutoCompleteFilterMode.Custom");
                 }
 
-                bool inResults = !(stringFiltering || objectFiltering);
-
-                if (!inResults)
+                foreach (object item in items)
                 {
-                    if (stringFiltering)
+                    // Exit the fitter when requested if cancellation is requested
+                    if (_cancelRequested)
                     {
-                        inResults = textFilter!(text, FormatValue(item));
+                        return;
                     }
-                    else if (objectFiltering)
+
+                    bool inResults = !(stringFiltering || objectFiltering);
+
+                    if (!inResults)
                     {
-                        inResults = itemFilter!(text, item);
+                        if (stringFiltering)
+                        {
+                            inResults = textFilter!(text, FormatValue(item));
+                        }
+                        else if (objectFiltering)
+                        {
+                            inResults = itemFilter!(text, item);
+                        }
                     }
-                }
 
-                if (inResults)
-                {
-                    _newViewItems.Add(item);
+                    if (inResults)
+                    {
+                        _newViewItems.Add(item);
+                    }
                 }
-            }
-
-            _view?.Clear();
-            _view?.AddRange(_newViewItems);
 
-            // Clear the evaluator to discard a reference to the last item
-            _valueBindingEvaluator?.ClearDataContext();
+                _view?.Clear();
+                _view?.AddRange(_newViewItems);
 
-            // indicate that filtering is not ongoing anymore
-            _filterInAction = false;
-            _cancelRequested = false;
+                // Clear the evaluator to discard a reference to the last item
+                _valueBindingEvaluator?.ClearDataContext();
+            }
+            finally
+            {
+                // indicate that filtering is not ongoing anymore
+                _filterInAction = false;
+                _cancelRequested = false;
+            }
         }
 
         /// <summary>

tests/Avalonia.Controls.UnitTests/AutoCompleteBoxTests.cs

@@ -495,6 +495,31 @@ namespace Avalonia.Controls.UnitTests
             });
         }
 
+        [Fact]
+        public void Attempting_To_Open_Without_Items_Does_Not_Prevent_Future_Opening_With_Items()
+        {
+            RunTest((control, textbox) =>
+            {
+                // Allow the drop down to open without anything entered.
+                control.MinimumPrefixLength = 0;
+
+                // Clear the items.
+                var source = control.ItemsSource;
+                control.ItemsSource = null;
+                control.IsDropDownOpen = true;
+
+                // DropDown was not actually opened because there are no items.
+                Assert.False(control.IsDropDownOpen);
+
+                // Set the items and try to open the drop down again.
+                control.ItemsSource = source;
+                control.IsDropDownOpen = true;
+
+                // DropDown can now be opened.
+                Assert.True(control.IsDropDownOpen);
+            });
+        }
+
         /// <summary>
         /// Retrieves a defined predicate filter through a new AutoCompleteBox 
         /// control instance.