Add support for running the Terminal without _any_ windows (#14944)

This adds a setting (`compatibility.allowHeadless`) to let the Terminal
keep running even when all windows are closed. This lets hotkeys keep
working, because the Emperor thread is still running, just, without any
windows.

I'm really tempted to invoke the magic "closes" word on #9996, but
honestly, we should also add some sort of support for `wt --headless` or
`wt --hidden` or whatever, before we close that. There's also #13630
which seems imminently doable.

Tested manually. I'd post a gif of "close all terminal windows, then
invoke the quakeMode binding and \*presto\*, but that would be an
unnecessarily big gif.

Related to #9996 but not enough to close it if you ask me

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

@@ -67,6 +67,7 @@ namespace winrt::TerminalApp::implementation
 
         Microsoft::Terminal::Settings::Model::Theme Theme();
         bool IsolatedMode();
+        bool AllowHeadless();
         bool RequestsTrayIcon();
 
         TerminalApp::TerminalWindow CreateNewWindow();

src/cascadia/TerminalApp/AppLogic.idl

@@ -46,6 +46,7 @@ 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

@@ -202,7 +202,13 @@ void WindowEmperor::_windowExitedHandler(uint64_t senderID)
                       return w->Peasant().GetID() == senderID;
                   });
 
-    if (_windowThreadInstances.fetch_sub(1, std::memory_order_relaxed) == 1)
+    // 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 quitWhenLastWindowExits{ !_app.Logic().AllowHeadless() };
+    const bool noMoreWindows{ _windowThreadInstances.fetch_sub(1, std::memory_order_relaxed) == 1 };
+    if (noMoreWindows &&
+        (_quitting || quitWhenLastWindowExits))
     {
         _close();
     }
@@ -297,6 +303,8 @@ 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

@@ -52,6 +52,8 @@ private:
 
     std::unique_ptr<NotificationIcon> _notificationIcon;
 
+    bool _quitting{ false };
+
     void _windowStartedHandlerPostXAML(const std::shared_ptr<WindowThread>& sender);
     void _windowExitedHandler(uint64_t senderID);