feat: add MicaAlt support for settings background (#17650)

doc/cascadia/profiles.schema.json

@@ -2018,6 +2018,12 @@
           "type": "boolean",
           "default": false
         },
+        "useMicaAlt": {
+        "description": "Enable the MicaAlt visual effect.",
+        "type": "boolean",
+        "default": false
+      },
+
         "experimental.rainbowFrame": {
           "description": "When enabled, the frame of the window will cycle through all the colors. Enabling this will override the `frame` and `unfocusedFrame` settings.",
           "type": "boolean",

src/cascadia/TerminalSettingsEditor/MainPage.cpp

@@ -981,7 +981,7 @@ namespace winrt::Microsoft::Terminal::Settings::Editor::implementation
         // that our theme is different than the app's.
         const bool actuallyUseMica = isMicaAvailable && (appTheme == requestedTheme);
 
-        const auto bgKey = (theme.Window() != nullptr && theme.Window().UseMica()) && actuallyUseMica ?
+        const auto bgKey = (theme.Window() != nullptr && theme.Window().UseMica()) || UseMicaAlt() && actuallyUseMica ?
                                L"SettingsPageMicaBackground" :
                                L"SettingsPageBackground";
 

src/cascadia/TerminalSettingsModel/GlobalAppSettings.cpp

@@ -35,6 +35,13 @@ static constexpr std::string_view LegacyConfirmCloseAllTabsKey{ "confirmCloseAll
 // - <none>
 // Return Value:
 // - <none>
+
+bool _useMicaAlt = false;
+
+bool GlobalAppSettings::UseMicaAlt() const
+{
+    return _useMicaAlt;
+}
 void GlobalAppSettings::_FinalizeInheritance()
 {
     for (const auto& parent : _parents)
@@ -208,6 +215,12 @@ void GlobalAppSettings::LayerJson(const Json::Value& json, const OriginTag origi
             }
         }
     }
+
+if (json.isMember("useMicaAlt"))
+    {
+    JsonUtils::GetValueForKey(json, "useMicaAlt", _useMicaAlt);
+    }
+
 }
 
 void GlobalAppSettings::LayerActionsFrom(const Json::Value& json, const OriginTag origin, const bool withKeybindings)

src/cascadia/TerminalSettingsModel/GlobalAppSettings.h

@@ -26,6 +26,9 @@ Author(s):
 #include "NewTabMenuEntry.h"
 #include "RemainingProfilesEntry.h"
 
+bool UseMicaAlt() const;
+bool _useMicaAlt = false;
+
 // fwdecl unittest classes
 namespace SettingsModelUnitTests
 {