Added conditional namespace for ShouldConstrainToRootBounds

Author: yoshiask

Microsoft.Toolkit.Uwp.UI.Controls.Core/TabbedCommandBar/TabbedCommandBar.xaml

@@ -1,6 +1,7 @@
 <ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
-                    xmlns:controls="using:Microsoft.Toolkit.Uwp.UI.Controls">
+                    xmlns:controls="using:Microsoft.Toolkit.Uwp.UI.Controls"
+                    xmlns:Windows10version1903="http://schemas.microsoft.com/winfx/2006/xaml/presentation?IsApiContractPresent(Windows.Foundation.UniversalApiContract, 8)">
 
     <ResourceDictionary.MergedDictionaries>
         <ResourceDictionary Source="ms-appx:///Microsoft.Toolkit.Uwp.UI.Controls.Core/TabbedCommandBar/TabbedCommandBarItem.xaml" />
@@ -88,7 +89,7 @@
                                                 Visibility="{Binding TemplateSettings.OverflowButtonVisibility, RelativeSource={RelativeSource Mode=TemplatedParent}}">
                                             <Button.Flyout>
                                                 <Flyout Placement="Bottom"
-                                                        ShouldConstrainToRootBounds="False">
+                                                        Windows10version1903:ShouldConstrainToRootBounds="False">
                                                     <Flyout.FlyoutPresenterStyle>
                                                         <Style TargetType="FlyoutPresenter">
                                                             <Setter Property="Padding" Value="0,8" />