Add a setting for the unfocused tabRow background color (#13049)

Adds `tabRow.unfocusedBackground` to the theme properties.

When provided, the window will use this ThemeColor as the color of the tab row when the window is inactive. 

When omitted, the window will fall back to the default tab row color, `{"key": "TabViewBackground"}` from our App.xaml.

* [ ] tests added.
* [x] Closes #4862
* [ ] Needs a whole pile of docs updates, which we'll do at the end here.


This actually helped validate #12992 quite a bit. I found a bunch of bugs concerning null colors, null objects. Json parsing is hard 😛

src/cascadia/TerminalApp/TerminalPage.cpp

@@ -4101,24 +4101,27 @@ namespace winrt::TerminalApp::implementation
 
             TitlebarBrush(acrylicBrush);
         }
-        else if (theme.TabRow() && theme.TabRow().Background())
+        else if (theme.TabRow())
         {
-            const auto tabRowBg = theme.TabRow().Background();
-            const auto terminalBrush = [this]() -> Media::Brush {
-                if (const auto& control{ _GetActiveControl() })
-                {
-                    return control.BackgroundBrush();
-                }
-                else if (auto settingsTab = _GetFocusedTab().try_as<TerminalApp::SettingsTab>())
-                {
-                    return settingsTab.Content().try_as<Settings::Editor::MainPage>().BackgroundBrush();
-                }
-                return nullptr;
-            }();
+            if (const auto tabRowBg{ _activated ? theme.TabRow().Background() :
+                                                  theme.TabRow().UnfocusedBackground() })
+            {
+                const auto terminalBrush = [this]() -> Media::Brush {
+                    if (const auto& control{ _GetActiveControl() })
+                    {
+                        return control.BackgroundBrush();
+                    }
+                    else if (auto settingsTab = _GetFocusedTab().try_as<TerminalApp::SettingsTab>())
+                    {
+                        return settingsTab.Content().try_as<Settings::Editor::MainPage>().BackgroundBrush();
+                    }
+                    return nullptr;
+                }();
 
-            const auto themeBrush{ tabRowBg.Evaluate(res, terminalBrush, true) };
-            bgColor = ThemeColor::ColorFromBrush(themeBrush);
-            TitlebarBrush(themeBrush);
+                const auto themeBrush{ tabRowBg.Evaluate(res, terminalBrush, true) };
+                bgColor = ThemeColor::ColorFromBrush(themeBrush);
+                TitlebarBrush(themeBrush);
+            }
         }
         else
         {

src/cascadia/TerminalSettingsModel/MTSMSettings.h

@@ -124,8 +124,9 @@ Author(s):
     X(winrt::Windows::UI::Xaml::ElementTheme, RequestedTheme, "applicationTheme", winrt::Windows::UI::Xaml::ElementTheme::Default) \
     X(bool, UseMica, "useMica", false)
 
-#define MTSM_THEME_TABROW_SETTINGS(X) \
-    X(winrt::Microsoft::Terminal::Settings::Model::ThemeColor, Background, "background", nullptr)
+#define MTSM_THEME_TABROW_SETTINGS(X)                                                             \
+    X(winrt::Microsoft::Terminal::Settings::Model::ThemeColor, Background, "background", nullptr) \
+    X(winrt::Microsoft::Terminal::Settings::Model::ThemeColor, UnfocusedBackground, "unfocusedBackground", nullptr)
 
 #define MTSM_THEME_TAB_SETTINGS(X) \
     X(winrt::Microsoft::Terminal::Settings::Model::ThemeColor, Background, "background", nullptr)

src/cascadia/TerminalSettingsModel/Theme.cpp

@@ -104,6 +104,10 @@ winrt::WUX::Media::Brush ThemeColor::Evaluate(const winrt::WUX::ResourceDictiona
     {
     case ThemeColorType::Accent:
     {
+        // NOTE: There is no canonical way to get the unfocused ACCENT titlebar
+        // color in Windows. Edge uses it's own heuristic, and in Windows 11,
+        // much of this logic is rapidly changing. We're not gonna mess with
+        // that, since it seems there's no good way to reverse engineer that.
         til::color accentColor = forTitlebar ?
                                      _getAccentColorForTitlebar() :
                                      til::color{ winrt::unbox_value<winrt::Windows::UI::Color>(res.Lookup(accentColorKey)) };

src/cascadia/TerminalSettingsModel/Theme.idl

@@ -34,6 +34,7 @@ namespace Microsoft.Terminal.Settings.Model
 
     runtimeclass TabRowTheme {
         ThemeColor Background { get; };
+        ThemeColor UnfocusedBackground { get; };
     }
 
     runtimeclass TabTheme {