Fixes for DataValidationErrors not working correctly with Bindings (issue #18693) (#18694)

* Added unit test for AutoCompleteBox DataValidationErrors
- checks that when the TextBox control's Errors are bound to the parent AutoCompleteBox, they get updated correctly in both parent AutoCompleteBox control and TextBox control

* Fix issue with DataValidationErrors causing issues with AutoCompleteBox because of Bindings
- when binding a control's DataValidationErrors.Errors property to the same property of another control, the use of a static bool flag "s_overridingErrors" was preventing the control with the Binding from being updated correctly
- issue is resolved by replacing static bool flag with a private AttachedProperty<bool> in DataValidationErrors.cs

* Clean up Asserts in previous AutoCompleteBoxTests.cs changes

Author: drone1400

src/Avalonia.Controls/DataValidationErrors.cs

@@ -18,7 +18,6 @@ namespace Avalonia.Controls
     [PseudoClasses(":error")]
     public class DataValidationErrors : ContentControl
     {
-        private static bool s_overridingErrors;
 
         /// <summary>
         /// Defines the DataValidationErrors.Errors attached property.
@@ -49,6 +48,12 @@ public class DataValidationErrors : ContentControl
         /// </summary>
         private static readonly AttachedProperty<IEnumerable<object>?> OriginalErrorsProperty =
             AvaloniaProperty.RegisterAttached<DataValidationErrors, Control, IEnumerable<object>?>("OriginalErrors");
+        
+        /// <summary>
+        /// Prevents executing ErrorsChanged after they are updated internally from OnErrorsOrConverterChanged
+        /// </summary>
+        private static readonly AttachedProperty<bool> OverridingErrorsInternallyProperty =
+            AvaloniaProperty.RegisterAttached<DataValidationErrors, Control, bool>("OverridingErrorsInternally", defaultValue: false);
 
         private Control? _owner;
 
@@ -96,9 +101,10 @@ public IDataTemplate ErrorTemplate
 
         private static void ErrorsChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            if (s_overridingErrors) return;
-
             var control = (Control)e.Sender;
+            
+            if (control.GetValue(OverridingErrorsInternallyProperty)) return;
+            
             var errors = (IEnumerable<object>?)e.NewValue;
 
             // Update original errors
@@ -140,14 +146,14 @@ private static void OnErrorsOrConverterChanged(Control control)
                     .Where(e => e is not null))?
                 .ToArray();
 
-            s_overridingErrors = true;
+            control.SetCurrentValue(OverridingErrorsInternallyProperty, true);
             try
             {
                 control.SetCurrentValue(ErrorsProperty, newErrors!);
             }
             finally
             {
-                s_overridingErrors = false;
+                control.SetCurrentValue(OverridingErrorsInternallyProperty, false);
             }
 
             control.SetValue(HasErrorsProperty, newErrors?.Any() == true);

tests/Avalonia.Controls.UnitTests/AutoCompleteBoxTests.cs

@@ -421,6 +421,28 @@ public void Text_Validation()
             });
         }
 
+        [Fact]
+        public void Text_Validation_TextBox_Errors_Binding()
+        {
+            RunTest((control, textbox) =>
+            {
+                // simulate the TemplateBinding that would be used within the AutoCompleteBox control theme for the inner PART_TextBox
+                //      DataValidationErrors.Errors="{TemplateBinding (DataValidationErrors.Errors)}"
+                textbox.Bind(DataValidationErrors.ErrorsProperty, control.GetBindingObservable(DataValidationErrors.ErrorsProperty));
+                
+                var exception = new InvalidCastException("failed validation");
+                var textObservable = new BehaviorSubject<BindingNotification>(new BindingNotification(exception, BindingErrorType.DataValidationError));
+                control.Bind(AutoCompleteBox.TextProperty, textObservable);
+                Dispatcher.UIThread.RunJobs();
+                
+                Assert.True(DataValidationErrors.GetHasErrors(control));
+                Assert.Equal([exception], DataValidationErrors.GetErrors(control));
+                
+                Assert.True(DataValidationErrors.GetHasErrors(textbox));
+                Assert.Equal([exception], DataValidationErrors.GetErrors(textbox));
+            });
+        }
+        
         [Fact]
         public void SelectedItem_Validation()
         {