Fix AutoCompleteBox not opening when the text box is empty

samples/ControlCatalog/Pages/AutoCompleteBoxPage.xaml

@@ -18,6 +18,10 @@
           <Setter Property="Margin" Value="8" />
         </Style>
       </UniformGrid.Styles>
+      <StackPanel>
+        <TextBlock Text="MinimumPrefixLength: 0" />
+        <AutoCompleteBox MinimumPrefixLength="0" />
+      </StackPanel>
       <StackPanel>
         <TextBlock Text="MinimumPrefixLength: 1" />
         <AutoCompleteBox MinimumPrefixLength="1" />
@@ -42,7 +46,6 @@
         <TextBlock Text="Disabled" />
         <AutoCompleteBox IsEnabled="False" />
       </StackPanel>
-
       <StackPanel>
         <TextBlock Text="ValueMemberBinding" />
         <AutoCompleteBox ValueMemberBinding="{Binding Capital, x:DataType=models:StateData}" />

src/Avalonia.Controls/AutoCompleteBox/AutoCompleteBox.cs

@@ -1320,17 +1320,14 @@ private void TextUpdated(string? newText, bool userInitiated)
             // Evaluate the conditions needed for completion.
             // 1. Minimum prefix length
             // 2. If a delay timer is in use, use it
-            bool populateReady = newText.Length >= MinimumPrefixLength && MinimumPrefixLength >= 0;
-            if (populateReady && MinimumPrefixLength == 0 && String.IsNullOrEmpty(newText) && String.IsNullOrEmpty(SearchText))
-            {
-                populateReady = false;
-            }
-            _userCalledPopulate = populateReady ? userInitiated : false;
+            bool minimumLengthReached = newText.Length >= MinimumPrefixLength && MinimumPrefixLength >= 0;
+
+            _userCalledPopulate = minimumLengthReached && userInitiated;
 
             // Update the interface and values only as necessary
             UpdateTextValue(newText, userInitiated);
 
-            if (populateReady)
+            if (minimumLengthReached)
             {
                 _ignoreTextSelectionChange = true;
 

tests/Avalonia.Controls.UnitTests/AutoCompleteBoxTests.cs

@@ -1,21 +1,15 @@
 using System;
-using System.Collections;
 using System.Collections.Generic;
-using System.ComponentModel;
 using System.Linq;
 using Avalonia.Controls.Primitives;
-using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Templates;
 using Avalonia.Data;
-using Avalonia.Markup.Data;
-using Avalonia.Platform;
 using Avalonia.Threading;
 using Avalonia.UnitTests;
-using Moq;
 using Xunit;
 using System.Collections.ObjectModel;
-using System.Reactive.Linq;
 using System.Reactive.Subjects;
+using Avalonia.Input;
 
 namespace Avalonia.Controls.UnitTests
 {
@@ -439,6 +433,29 @@ public void SelectedItem_Validation()
             });
         }
 
+        [Fact]
+        public void Explicit_Dropdown_Open_Request_MinimumPrefixLength_0()
+        {
+            RunTest((control, textbox) =>
+            {
+                control.Text = "";
+                control.MinimumPrefixLength = 0;
+                Dispatcher.UIThread.RunJobs();
+
+                Assert.False(control.IsDropDownOpen);
+
+                control.RaiseEvent(new KeyEventArgs
+                {
+                    RoutedEvent = InputElement.KeyDownEvent,
+                    Key = Key.Down
+                });
+
+                Dispatcher.UIThread.RunJobs();
+
+                Assert.True(control.IsDropDownOpen);
+            });
+        }
+
         /// <summary>
         /// Retrieves a defined predicate filter through a new AutoCompleteBox 
         /// control instance.
@@ -1072,14 +1089,14 @@ private void RunTest(Action<AutoCompleteBox, TextBox> test)
 
         private AutoCompleteBox CreateControl()
         {
-            var datePicker =
+            var autoCompleteBox =
                 new AutoCompleteBox
                 {
                     Template = CreateTemplate()
                 };
 
-            datePicker.ApplyTemplate();
-            return datePicker;
+            autoCompleteBox.ApplyTemplate();
+            return autoCompleteBox;
         }
         private TextBox GetTextBox(AutoCompleteBox control)
         {