Add the background back to showTabsInTitlebar: false's tab row. (#1…

…3144)

We're doing it this way because ThemeResources are tricky. We
default in XAML to using the appropriate ThemeResource background
color for our TabRow. When tabs in the titlebar are _disabled_,
this will ensure that the tab row has the correct theme-dependent
value. When tabs in the titlebar are _enabled_ (the default),
we'll switch the BG to Transparent, to let the Titlebar Control's
background be used as the BG for the tab row.

We can't do it the other way around (default to Transparent, only
switch to a color when disabling tabs in the titlebar), because
looking up the correct ThemeResource from and App dictionary is a
capital-H Hard problem.

* [x] Closes #13143
* [x] I work here
* [x] validated manually:
  - [x] showTabsInTitlebar: false, true
  - [x] useAcrylicInTabRow: false, true
  - [x] theme: light, dark
* [x] Need to check if this is regressed the same in 1.13. I suspect it is.

(cherry picked from commit 4c33353)
Service-Card-Id: 82298857
Service-Version: 1.13

src/cascadia/TerminalApp/TabRowControl.xaml

@@ -9,8 +9,11 @@
                   xmlns:local="using:TerminalApp"
                   xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
                   xmlns:mux="using:Microsoft.UI.Xaml.Controls"
+                  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.  -->
+
     <mux:TabView x:Name="TabView"
                  VerticalAlignment="Bottom"
                  HorizontalContentAlignment="Stretch"

src/cascadia/TerminalApp/TerminalPage.cpp

@@ -211,6 +211,24 @@ namespace winrt::TerminalApp::implementation
 
             // Inform the host that our titlebar content has changed.
             _SetTitleBarContentHandlers(*this, _tabRow);
+
+            // GH#13143 Manually set the tab row's background to transparent here.
+            //
+            // We're doing it this way because ThemeResources are tricky. We
+            // default in XAML to using the appropriate ThemeResource background
+            // color for our TabRow. When tabs in the titlebar are _disabled_,
+            // this will ensure that the tab row has the correct theme-dependent
+            // value. When tabs in the titlebar are _enabled_ (the default),
+            // we'll switch the BG to Transparent, to let the Titlebar Control's
+            // background be used as the BG for the tab row.
+            //
+            // We can't do it the other way around (default to Transparent, only
+            // switch to a color when disabling tabs in the titlebar), because
+            // looking up the correct ThemeResource from and App dictionary is a
+            // capital-H Hard problem.
+            const auto transparent = Media::SolidColorBrush();
+            transparent.Color(Windows::UI::Colors::Transparent());
+            _tabRow.Background(transparent);
         }
 
         // Hookup our event handlers to the ShortcutActionDispatch