Prevent RPC_E_CANTCALLOUT_ININPUTSYNCCALL during WM_SETTINGCHANGE (#19755)

Closes #19505

(cherry picked from commit e8971c8)
Service-Card-Id: PVTI_lADOAF3p4s4AxadtzgkC_Mk
Service-Version: 1.23

Author: lhecker

.github/actions/spelling/expect/expect.txt

@@ -153,6 +153,7 @@ cac
 cacafire
 CALLCONV
 CANDRABINDU
+CANTCALLOUT
 capslock
 CARETBLINKINGENABLED
 CARRIAGERETURN
@@ -858,6 +859,7 @@ inclusivity
 INCONTEXT
 INFOEX
 inheritcursor
+ININPUTSYNCCALL
 INITCOMMONCONTROLSEX
 INITDIALOG
 initguid

src/cascadia/TerminalApp/AppLogic.cpp

@@ -150,7 +150,9 @@ namespace winrt::TerminalApp::implementation
             });
 
         _languageProfileNotifier = winrt::make_self<LanguageProfileNotifier>([this]() {
-            _reloadSettings->Run();
+            // TODO: This is really bad, because we reset any current user customizations.
+            // See GH#11522.
+            ReloadSettingsThrottled();
         });
 
         // Do this here, rather than at the top of main. This will prevent us from
@@ -330,7 +332,7 @@ namespace winrt::TerminalApp::implementation
 
                 if (modifiedBasename == settingsBasename)
                 {
-                    _reloadSettings->Run();
+                    ReloadSettingsThrottled();
                 }
             });
     }
@@ -436,6 +438,11 @@ namespace winrt::TerminalApp::implementation
         SettingsChanged.raise(*this, *ev);
     }
 
+    void AppLogic::ReloadSettingsThrottled()
+    {
+        _reloadSettings->Run();
+    }
+
     // This is a continuation of AppLogic::Create() and includes the more expensive parts.
     void AppLogic::NotifyRootInitialized()
     {

src/cascadia/TerminalApp/AppLogic.h

@@ -36,6 +36,7 @@ namespace winrt::TerminalApp::implementation
         bool IsRunningElevated() const noexcept;
         bool CanDragDrop() const noexcept;
         void ReloadSettings();
+        void ReloadSettingsThrottled();
         void NotifyRootInitialized();
 
         bool HasSettingsStartupActions() const noexcept;
@@ -80,7 +81,6 @@ namespace winrt::TerminalApp::implementation
         [[nodiscard]] HRESULT _TryLoadSettings() noexcept;
         void _ProcessLazySettingsChanges();
         void _RegisterSettingsChange();
-        safe_void_coroutine _DispatchReloadSettings();
 
         void _setupFolderPathEnvVar();
 

src/cascadia/TerminalApp/AppLogic.idl

@@ -25,6 +25,7 @@ namespace TerminalApp
         Boolean HasSettingsStartupActions();
 
         void ReloadSettings();
+        void ReloadSettingsThrottled();
         Microsoft.Terminal.Settings.Model.CascadiaSettings Settings { get; };
 
         TerminalWindow CreateNewWindow();

src/cascadia/WindowsTerminal/WindowEmperor.cpp

@@ -991,7 +991,13 @@ LRESULT WindowEmperor::_messageHandler(HWND window, UINT const message, WPARAM c
                 if (isCurrentlyDark != _currentSystemThemeIsDark)
                 {
                     _currentSystemThemeIsDark = isCurrentlyDark;
-                    _app.Logic().ReloadSettings();
+
+                    // GH#19505: WM_SETTINGCHANGE gets sent out with a SendMessage() call, which means
+                    // that COM methods marked as [input_sync] cannot be called. Well, our CascadiaSettings
+                    // loader does call such methods. This results in RPC_E_CANTCALLOUT_ININPUTSYNCCALL, aka:
+                    // "An outgoing call cannot be made since the application is dispatching an input-synchronous call."
+                    // The solution is to simply do it in another tick.
+                    _app.Logic().ReloadSettingsThrottled();
                 }
             }
             return 0;