feat: add avalonia FlyoutPresenter style.

Author: NaBian

src/Avalonia/HandyControl_Avalonia/Themes/Styles/FlyoutPresenter.axaml

@@ -0,0 +1,50 @@
+<ResourceDictionary xmlns="https://github.com/avaloniaui"
+                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                    x:ClassModifier="internal"
+                    xmlns:hc="https://handyorg.github.io/handycontrol">
+
+    <ControlTheme x:Key="{x:Type FlyoutPresenter}"
+                  TargetType="FlyoutPresenter">
+        <Setter Property="HorizontalContentAlignment"
+                Value="Stretch" />
+        <Setter Property="VerticalContentAlignment"
+                Value="Stretch" />
+        <Setter Property="Background"
+                Value="{DynamicResource RegionBrush}" />
+        <Setter Property="BorderBrush"
+                Value="{DynamicResource BorderBrush}" />
+        <Setter Property="BorderThickness"
+                Value="1" />
+        <Setter Property="ScrollViewer.HorizontalScrollBarVisibility"
+                Value="Auto" />
+        <Setter Property="ScrollViewer.VerticalScrollBarVisibility"
+                Value="Auto" />
+        <Setter Property="hc:BorderElement.CornerRadius"
+                Value="{StaticResource DefaultCornerRadius}" />
+        <Setter Property="CornerRadius"
+                Value="{Binding $self.(hc:BorderElement.CornerRadius)}" />
+        <Setter Property="Template">
+            <ControlTemplate>
+                <Border Effect="{StaticResource EffectShadow2}"
+                        Margin="8"
+                        Background="{TemplateBinding Background}"
+                        BorderBrush="{TemplateBinding BorderBrush}"
+                        BorderThickness="{TemplateBinding BorderThickness}"
+                        CornerRadius="{TemplateBinding CornerRadius}">
+                    <ScrollViewer HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}"
+                                  VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}">
+                        <ContentPresenter x:Name="PART_ContentPresenter"
+                                          Margin="{TemplateBinding Padding}"
+                                          HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
+                                          VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
+                                          HorizontalContentAlignment="Stretch"
+                                          VerticalContentAlignment="Stretch"
+                                          Content="{TemplateBinding Content}"
+                                          ContentTemplate="{TemplateBinding ContentTemplate}" />
+                    </ScrollViewer>
+                </Border>
+            </ControlTemplate>
+        </Setter>
+    </ControlTheme>
+
+</ResourceDictionary>

src/Avalonia/HandyControl_Avalonia/Themes/Styles/MergedStyles.axaml

@@ -17,6 +17,7 @@
 
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/AdornerLayer.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/DataValidationErrors.axaml" />
+                <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/FlyoutPresenter.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/MenuFlyoutPresenter.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/TransitioningContentControl.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/PopupRoot.axaml" />