Remove a 1px gap under the tabs only visible at >150% (#15164)

Set the padding to the default TabViewHeaderPadding (8,0,0,0), but with
-1 on the bottom. This prevents a small 1px gap that can appear on 150%
scale displays between the tab item and the content. The 1 on top helps
keep
the tab the correct relative height within the tab row.


Regressed in #15078 

See also MSFT:40692364

src/cascadia/TerminalApp/App.xaml

@@ -46,6 +46,7 @@
 
                     <!--  Suppress top padding  -->
                     <Thickness x:Key="TabViewHeaderPadding">9,0,5,0</Thickness>
+
                     <Thickness x:Key="TabViewItemBorderThickness">1,1,1,0</Thickness>
 
                     <!--

src/cascadia/TerminalApp/MinMaxCloseControl.xaml

@@ -145,8 +145,8 @@
                 details.
             -->
             <x:Double x:Key="CaptionButtonHeightWindowed">40.0</x:Double>
-            <!--  32 + 1 to compensate for GH#10746  -->
-            <x:Double x:Key="CaptionButtonHeightMaximized">33.0</x:Double>
+            <!--  32 + (1 to compensate for GH#10746) + (-1 for GH#15164)  -->
+            <x:Double x:Key="CaptionButtonHeightMaximized">32.0</x:Double>
 
             <Style x:Key="CaptionButton"
                    TargetType="Button">

src/cascadia/TerminalApp/TabRowControl.xaml

@@ -12,9 +12,20 @@
                   Background="{ThemeResource TabViewBackground}"
                   mc:Ignorable="d">
 
-    <!--  GH#13143: Make sure that the Background is actually TabViewBackground here, not Transparent. This is load bearing, for showTabsInTitlebar=false.  -->
+    <!--
+        GH#13143: Make sure that the Background is actually TabViewBackground
+        here, not Transparent. This is load bearing, for showTabsInTitlebar=false.
+    -->
+
+    <!--
+        Set the padding to -1 on the bottom. This prevents a small 1px gap that
+        can appear on 150% scale displays between the tab item and the content.
+        The 1 on top helps keep the tab the correct relative height within the
+        tab row.
+    -->
 
     <mux:TabView x:Name="TabView"
+                 Padding="0,1,0,-1"
                  VerticalAlignment="Bottom"
                  HorizontalContentAlignment="Stretch"
                  AllowDropTabs="True"