popup can't be bottom case

Author: mou-haz

source/iNKORE.UI.WPF.Modern/Controls/Helpers/WinUIComboBoxBehaviorHelper.cs

@@ -98,8 +98,13 @@ private static void OnDropDownOpened(object sender, object args)
 
         private static void AlignSelectedContainer(ComboBox comboBox, Popup popup, bool isOpenDown)
         {
-            if (!isOpenDown ||
-                GetToAlignContainer(comboBox) is not { } itemContainer ||
+            if (comboBox.IsEditable)
+            {
+                popup.VerticalOffset = 0;
+                return;
+            }
+            
+            if (GetToAlignContainer(comboBox, isOpenDown) is not { } itemContainer ||
                 itemContainer.TranslatePoint(new Point(0, -itemContainer.ActualHeight + comboBox.Padding.Top),
                     comboBox) is not { Y: not 0 } itemTop)
             {
@@ -114,27 +119,20 @@ private static void AlignSelectedContainer(ComboBox comboBox, Popup popup, bool
             }
         }
 
-        private static FrameworkElement GetToAlignContainer(ComboBox comboBox)
+        private static FrameworkElement GetToAlignContainer(ComboBox comboBox, bool isOpenDown)
         {
             DependencyObject container;
-            if (comboBox.SelectedItem is null)
+            if (!isOpenDown)
+            {
+                //get container at the bottom of viewport
+                //ComboBox ItemContainerGenerator block size is 15
+                container = comboBox.ItemContainerGenerator.ContainerFromIndex(Math.Min(15, comboBox.Items.Count) - 1);
+            }
+            else if (comboBox.SelectedItem is null)
             {
                 container = comboBox.ItemContainerGenerator.ContainerFromIndex(
                     (int)Math.Ceiling(comboBox.Items.Count / 2.0));
-
-                if (comboBox.ItemContainerGenerator.ContainerFromIndex(0) is ComboBoxItem item)
-                {
-                    var highlightedInfoProperty = typeof(ComboBox).GetProperty("HighlightedInfo",
-                        BindingFlags.Instance | BindingFlags.NonPublic);
-
-                    var setter = highlightedInfoProperty.SetMethod;
-
-                    var itemInfo = typeof(ComboBox)
-                        .GetMethod("ItemInfoFromContainer", BindingFlags.Instance | BindingFlags.NonPublic)?
-                        .Invoke(comboBox, [item]);
-
-                    setter?.Invoke(comboBox, [itemInfo]);
-                }
+                TryHighlightingFirstItem(comboBox);
             }
             else
             {
@@ -144,6 +142,25 @@ private static FrameworkElement GetToAlignContainer(ComboBox comboBox)
             return container as FrameworkElement;
         }
 
+        private static void TryHighlightingFirstItem(ComboBox comboBox)
+        {
+            if (comboBox.ItemContainerGenerator.ContainerFromIndex(0) is not ComboBoxItem item)
+            {
+                return;
+            }
+
+            var highlightedInfoProperty = typeof(ComboBox).GetProperty("HighlightedInfo",
+                BindingFlags.Instance | BindingFlags.NonPublic);
+
+            var setter = highlightedInfoProperty?.SetMethod;
+
+            var itemInfo = typeof(ComboBox)
+                .GetMethod("ItemInfoFromContainer", BindingFlags.Instance | BindingFlags.NonPublic)?
+                .Invoke(comboBox, [item]);
+
+            setter?.Invoke(comboBox, [itemInfo]);
+        }
+
         private static void UpdateCornerRadius(ComboBox comboBox, Popup? popup, bool isDropDownOpen, bool isOpenDown)
         {
             var textBoxRadius = ControlHelper.GetCornerRadius(comboBox);
@@ -194,17 +211,15 @@ private static CornerRadius GetFilteredPopupRadius(CornerRadius popupRadius, boo
 
         private static bool IsPopupOpenDown(ComboBox comboBox, double popupVerticalOffset)
         {
-            double verticalOffset = popupVerticalOffset;
-            if (GetTemplateChild<Border>(c_popupBorderName, comboBox) is { } popupBorder)
+            if (GetTemplateChild<Border>(c_popupBorderName, comboBox) is not { } popupBorder ||
+                GetTemplateChild<TextBox>(c_editableTextName, comboBox) is not { } textBox)
             {
-                if (GetTemplateChild<TextBox>(c_editableTextName, comboBox) is { } textBox)
-                {
-                    var popupTop = popupBorder.TranslatePoint(new Point(0, 0), textBox);
-                    verticalOffset = popupTop.Y;
-                }
+                return false;
             }
-
-            return verticalOffset > popupVerticalOffset;
+            
+            var popupTopPoint = popupBorder.TranslatePoint(new Point(0, 0), textBox);
+            
+            return popupTopPoint.Y + popupVerticalOffset > 0;
         }
 
         private static object ResourceLookup(Control control, object key)