titlebar / caption buttons use same template in default theme.

src/Avalonia.Themes.Default/CaptionButtons.xaml

@@ -1,11 +1,6 @@
 <Styles xmlns="https://github.com/avaloniaui" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
-  <Design.PreviewWith>
-    <Border Padding="20" Background="Black">
-      <CaptionButtons Height="30" Foreground="{DynamicResource SystemControlForegroundBaseHighBrush}" />
-    </Border>
-  </Design.PreviewWith>
-  <Style Selector="CaptionButtons">    
-    <Setter Property="Foreground" Value="{TemplateBinding Foreground}"/>
+  <Style Selector="CaptionButtons">
+    <Setter Property="Foreground" Value="{DynamicResource ThemeForegroundBrush}"/>
     <Setter Property="Canvas.Right" Value="0" />
     <Setter Property="MaxHeight" Value="30" />
     <Setter Property="Template">
@@ -16,39 +11,39 @@
               <Setter Property="Width" Value="45" />
               <Setter Property="Background" Value="Transparent" />
             </Style>
-            <Style Selector="Panel:pointerover.captionButton">
-              <Setter Property="Background" Value="Red" />
+            <Style Selector="Panel:pointerover">
+              <Setter Property="Background" Value="#7F7f7f7f" />
             </Style>
             <Style Selector="Panel#PART_CloseButton:pointerover">
-              <Setter Property="Background" Value="#AFFF0000" />
+              <Setter Property="Background" Value="#7FFF0000" />
             </Style>
             <Style Selector="Viewbox">
               <Setter Property="Width" Value="11" />
               <Setter Property="Margin" Value="2" />
             </Style>
           </StackPanel.Styles>
-          <Panel x:Name="PART_FullScreenButton" Classes="captionButton">
+          <Panel x:Name="PART_FullScreenButton">
             <Viewbox>
               <Path Stretch="UniformToFill" Fill="{TemplateBinding Foreground}" />
             </Viewbox>
           </Panel>
 
-          <Panel x:Name="PART_MinimiseButton"  Classes="captionButton">
+          <Panel x:Name="PART_MinimiseButton">
             <Viewbox>
               <Path Stretch="UniformToFill" Fill="{TemplateBinding Foreground}" Data="M2048 1229v-205h-2048v205h2048z" />
             </Viewbox>
           </Panel>
 
-          <Panel x:Name="PART_RestoreButton"  Classes="captionButton">
+          <Panel x:Name="PART_RestoreButton">
             <Viewbox>
               <Viewbox.RenderTransform>
                 <RotateTransform Angle="-90" />
               </Viewbox.RenderTransform>
-              <Path Stretch="UniformToFill" Fill="{TemplateBinding Foreground}"/>              
+              <Path Stretch="UniformToFill" Fill="{TemplateBinding Foreground}"/>
             </Viewbox>
           </Panel>
 
-          <Panel x:Name="PART_CloseButton"  Classes="captionButton">
+          <Panel x:Name="PART_CloseButton">
             <Viewbox>
               <Path Stretch="UniformToFill" Fill="{TemplateBinding Foreground}" Data="M1169 1024l879 -879l-145 -145l-879 879l-879 -879l-145 145l879 879l-879 879l145 145l879 -879l879 879l145 -145z" />
             </Viewbox>
@@ -69,4 +64,7 @@
   <Style Selector="CaptionButtons:fullscreen Panel#PART_FullScreenButton Path">
     <Setter Property="Data" Value="M205 1024h819v-819h-205v469l-674 -674l-145 145l674 674h-469v205zM1374 1229h469v-205h-819v819h205v-469l674 674l145 -145z" />
   </Style>
-</Styles>
+  <Style Selector="CaptionButtons:fullscreen Panel#PART_RestoreButton, CaptionButtons:fullscreen Panel#PART_MinimiseButton">
+    <Setter Property="IsVisible" Value="False" />
+  </Style>
+ </Styles>

src/Avalonia.Themes.Default/TitleBar.xaml

@@ -5,15 +5,49 @@
     </Border>
   </Design.PreviewWith>
   <Style Selector="TitleBar">
-    <Setter Property="Foreground" Value="{DynamicResource ThemeForegroundBrush}"/>    
-    <Setter Property="MaxHeight" Value="30" />
-    <Setter Property="Background" Value="Red" />
+    <Setter Property="Foreground" Value="{DynamicResource SystemControlForegroundBaseHighBrush}"/>    
+    <Setter Property="VerticalAlignment" Value="Top" />
+    <Setter Property="HorizontalAlignment" Value="Stretch" />
+    <Setter Property="Background" Value="Transparent" />
     <Setter Property="Template">
       <ControlTemplate>
-        <Border Background="{TemplateBinding Background}" HorizontalAlignment="Stretch">
-          <CaptionButtons Name="PART_CaptionButtons" HorizontalAlignment="Right" Foreground="{TemplateBinding Foreground}" />
-        </Border>
+        <Panel HorizontalAlignment="{TemplateBinding HorizontalAlignment}" VerticalAlignment="Stretch">
+          <Panel x:Name="PART_MouseTracker" Height="1" VerticalAlignment="Top" />
+          <Panel x:Name="PART_Container">
+            <Border x:Name="PART_Background" Background="{TemplateBinding Background}" />
+            <CaptionButtons x:Name="PART_CaptionButtons" VerticalAlignment="Top" HorizontalAlignment="Right" Foreground="{TemplateBinding Foreground}" MaxHeight="30" />
+          </Panel>
+        </Panel>
       </ControlTemplate>
     </Setter>
   </Style>
+
+  <Style Selector="TitleBar:fullscreen">
+    <Setter Property="Background" Value="{DynamicResource SystemAccentColor}" />
+  </Style>
+
+  <Style Selector="TitleBar /template/ Border#PART_Background">
+    <Setter Property="IsHitTestVisible" Value="False" />
+  </Style>
+
+  <Style Selector="TitleBar:fullscreen /template/ Border#PART_Background">
+    <Setter Property="IsHitTestVisible" Value="True" />
+  </Style>
+
+  <Style Selector="TitleBar:fullscreen /template/ Panel#PART_MouseTracker">
+    <Setter Property="Background" Value="Transparent" />
+  </Style>
+
+  <Style Selector="TitleBar:fullscreen /template/ Panel#PART_Container">
+    <Setter Property="RenderTransform" Value="translateY(-30px)" />
+    <Setter Property="Transitions">
+      <Transitions>
+        <TransformOperationsTransition Property="RenderTransform" Duration="0:0:.25" />
+      </Transitions>
+    </Setter>
+  </Style>
+
+  <Style Selector="TitleBar:fullscreen:pointerover /template/ Panel#PART_Container">
+    <Setter Property="RenderTransform" Value="none" />
+  </Style>
 </Styles>