Add support for running the Terminal without _any_ windows

src/cascadia/TerminalApp/AppLogic.cpp

@@ -703,6 +703,16 @@ namespace winrt::TerminalApp::implementation
                globals.MinimizeToNotificationArea();
     }
 
+    bool AppLogic::AllowHeadless()
+    {
+        if (!_loadedInitialSettings)
+        {
+            // Load settings if we haven't already
+            ReloadSettings();
+        }
+        return _settings.GlobalSettings().AllowHeadless();
+    }
+
     TerminalApp::TerminalWindow AppLogic::CreateNewWindow()
     {
         if (_settings == nullptr)

src/cascadia/TerminalApp/AppLogic.h

@@ -66,6 +66,7 @@ namespace winrt::TerminalApp::implementation
 
         Microsoft::Terminal::Settings::Model::Theme Theme();
         bool IsolatedMode();
+        bool AllowHeadless();
         bool RequestsTrayIcon();
 
         TerminalApp::TerminalWindow CreateNewWindow();

src/cascadia/TerminalApp/AppLogic.idl

@@ -44,6 +44,7 @@ namespace TerminalApp
         // Selected settings to expose
         Microsoft.Terminal.Settings.Model.Theme Theme { get; };
         Boolean IsolatedMode { get; };
+        Boolean AllowHeadless { get; };
         Boolean RequestsTrayIcon { get; };
 
         FindTargetWindowResult FindTargetWindow(String[] args);

src/cascadia/TerminalSettingsModel/GlobalAppSettings.idl

@@ -99,6 +99,7 @@ namespace Microsoft.Terminal.Settings.Model
         INHERITABLE_SETTING(IVector<NewTabMenuEntry>, NewTabMenu);
         INHERITABLE_SETTING(Boolean, EnableColorSelection);
         INHERITABLE_SETTING(Boolean, IsolatedMode);
+        INHERITABLE_SETTING(Boolean, AllowHeadless);
 
         Windows.Foundation.Collections.IMapView<String, ColorScheme> ColorSchemes();
         void AddColorScheme(ColorScheme scheme);

src/cascadia/TerminalSettingsModel/MTSMSettings.h

@@ -64,6 +64,7 @@ Author(s):
     X(bool, TrimPaste, "trimPaste", true)                                                                                                                                                             \
     X(bool, EnableColorSelection, "experimental.enableColorSelection", false)                                                                                                                         \
     X(winrt::Windows::Foundation::Collections::IVector<Model::NewTabMenuEntry>, NewTabMenu, "newTabMenu", winrt::single_threaded_vector<Model::NewTabMenuEntry>({ Model::RemainingProfilesEntry{} })) \
+    X(bool, AllowHeadless, "compatibility.allowHeadless", false)                                                                                                                                      \
     X(bool, IsolatedMode, "compatibility.isolatedMode", false)
 
 #define MTSM_PROFILE_SETTINGS(X)                                                                                                                               \

src/cascadia/WindowsTerminal/WindowEmperor.cpp

@@ -181,7 +181,13 @@ void WindowEmperor::_windowExitedHandler(uint64_t senderID)
                       return w->Peasant().GetID() == senderID;
                   });
 
-    if (lockedWindows->size() == 0)
+    // When we run out of windows, exit our process if and only if:
+    // * We're not allowed to run headless OR
+    // * we've explicitly been told to "quit", which should fully exit the Terminal.
+    const bool noMoreWindows{ lockedWindows->size() == 0 };
+    const bool quitWhenLastWindowExits{ !_app.Logic().AllowHeadless() };
+    if (noMoreWindows &&
+        (_quitting || quitWhenLastWindowExits))
     {
         _close();
     }
@@ -276,6 +282,8 @@ void WindowEmperor::_numberOfWindowsChanged(const winrt::Windows::Foundation::II
 void WindowEmperor::_quitAllRequested(const winrt::Windows::Foundation::IInspectable&,
                                       const winrt::Microsoft::Terminal::Remoting::QuitAllRequestedArgs& args)
 {
+    _quitting = true;
+
     // Make sure that the current timer is destroyed so that it doesn't attempt
     // to run while we are in the middle of quitting.
     if (_getWindowLayoutThrottler.has_value())

src/cascadia/WindowsTerminal/WindowEmperor.h

@@ -51,6 +51,8 @@ class WindowEmperor : public std::enable_shared_from_this<WindowEmperor>
 
     std::unique_ptr<NotificationIcon> _notificationIcon;
 
+    bool _quitting{ false };
+
     void _windowStartedHandler(const std::shared_ptr<WindowThread>& sender);
     void _windowExitedHandler(uint64_t senderID);