Run the save on a background thread so that we dont block the ui.

src/cascadia/WindowsTerminal/AppHost.cpp

@@ -87,19 +87,9 @@ AppHost::AppHost() noexcept :
     _window->MakeWindow();
 
     _windowManager.GetWindowLayoutRequested([this](auto&&, const winrt::Microsoft::Terminal::Remoting::GetWindowLayoutArgs& args) {
-        if (_windowManager.IsMonarch())
-        {
-            // If we are the monarch then the peasant thread is on the ui thread
-            // and we can just ask directly for the layout.
-            const auto pos = _GetWindowLaunchPosition();
-            args.WindowLayoutJson(_logic.GetWindowLayoutJson(pos));
-        }
-        else
-        {
-            // Otherwise they are on a separate thread, and we need to switch
-            // contexts to be able to get the json.
-            args.WindowLayoutJsonAsync(_GetWindowLayoutAsync());
-        }
+        // The peasants are running on separate threads, so they'll need to
+        // swap what context they are in to the ui thread to get the actual layout.
+        args.WindowLayoutJsonAsync(_GetWindowLayoutAsync());
     });
 
     _windowManager.BecameMonarch({ this, &AppHost::_BecomeMonarch });
@@ -710,7 +700,8 @@ void AppHost::_DispatchCommandline(winrt::Windows::Foundation::IInspectable send
 // - Asynchronously get the window layout from the current page. This is
 //   done async because we need to switch between the ui thread and the calling
 //   thread.
-// - NB: This will fail if called on the monarch, it should only be used for a peasant.
+// - NB: The peasant calling this must not be running on the UI thread, otherwise
+//   they will crash since they just call .get on the async operation.
 // Arguments:
 // - <none>
 // Return Value:
@@ -810,16 +801,15 @@ void AppHost::_BecomeMonarch(const winrt::Windows::Foundation::IInspectable& /*s
 
 winrt::fire_and_forget AppHost::_SaveWindowLayouts()
 {
-    // Use the main thread since we are accessing controls.
-    co_await winrt::resume_foreground(_logic.GetRoot().Dispatcher());
+    // Make sure we run on a background thread to not block anything.
+    co_await winrt::resume_background();
 
     if (_logic.ShouldUsePersistedLayout())
     {
         auto layoutJsons = _windowManager.GetAllWindowLayouts();
         _logic.SaveWindowLayoutJsons(layoutJsons);
     }
 
-    co_await winrt::resume_background();
     if (_getWindowLayoutThrottler.has_value())
     {
         // don't need to save too frequently