Fix ComboBox popup content to allow grouping in Fluent (#10878)

Author: h3xds1nz

src/Microsoft.DotNet.Wpf/src/Themes/PresentationFramework.Fluent/Styles/ComboBox.xaml

@@ -282,8 +282,7 @@
                                                 TextElement.FontWeight="{TemplateBinding FontWeight}"
                                                 TextElement.Foreground="{TemplateBinding Foreground}"
                                                 VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}">
-                                                <StackPanel
-                                                    IsItemsHost="True"
+                                                <ItemsPresenter
                                                     KeyboardNavigation.DirectionalNavigation="Contained"
                                                     TextElement.FontSize="{TemplateBinding FontSize}" />
                                             </ScrollViewer>

src/Microsoft.DotNet.Wpf/src/Themes/PresentationFramework.Fluent/Themes/Fluent.Dark.xaml

@@ -1551,7 +1551,7 @@
                     </Border.RenderTransform>
                     <Grid>
                       <ScrollViewer MaxHeight="{TemplateBinding MaxDropDownHeight}" Margin="0" HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}" SnapsToDevicePixels="True" TextElement.FontSize="{TemplateBinding FontSize}" TextElement.FontWeight="{TemplateBinding FontWeight}" TextElement.Foreground="{TemplateBinding Foreground}" VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}">
-                        <StackPanel IsItemsHost="True" KeyboardNavigation.DirectionalNavigation="Contained" TextElement.FontSize="{TemplateBinding FontSize}" />
+                        <ItemsPresenter KeyboardNavigation.DirectionalNavigation="Contained" TextElement.FontSize="{TemplateBinding FontSize}" />
                       </ScrollViewer>
                     </Grid>
                     <Border.Effect>

src/Microsoft.DotNet.Wpf/src/Themes/PresentationFramework.Fluent/Themes/Fluent.HC.xaml

@@ -1464,7 +1464,7 @@
                     </Border.RenderTransform>
                     <Grid>
                       <ScrollViewer MaxHeight="{TemplateBinding MaxDropDownHeight}" Margin="0" HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}" SnapsToDevicePixels="True" TextElement.FontSize="{TemplateBinding FontSize}" TextElement.FontWeight="{TemplateBinding FontWeight}" TextElement.Foreground="{TemplateBinding Foreground}" VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}">
-                        <StackPanel IsItemsHost="True" KeyboardNavigation.DirectionalNavigation="Contained" TextElement.FontSize="{TemplateBinding FontSize}" />
+                        <ItemsPresenter KeyboardNavigation.DirectionalNavigation="Contained" TextElement.FontSize="{TemplateBinding FontSize}" />
                       </ScrollViewer>
                     </Grid>
                     <Border.Effect>

src/Microsoft.DotNet.Wpf/src/Themes/PresentationFramework.Fluent/Themes/Fluent.Light.xaml

@@ -1560,7 +1560,7 @@
                     </Border.RenderTransform>
                     <Grid>
                       <ScrollViewer MaxHeight="{TemplateBinding MaxDropDownHeight}" Margin="0" HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}" SnapsToDevicePixels="True" TextElement.FontSize="{TemplateBinding FontSize}" TextElement.FontWeight="{TemplateBinding FontWeight}" TextElement.Foreground="{TemplateBinding Foreground}" VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}">
-                        <StackPanel IsItemsHost="True" KeyboardNavigation.DirectionalNavigation="Contained" TextElement.FontSize="{TemplateBinding FontSize}" />
+                        <ItemsPresenter KeyboardNavigation.DirectionalNavigation="Contained" TextElement.FontSize="{TemplateBinding FontSize}" />
                       </ScrollViewer>
                     </Grid>
                     <Border.Effect>