When using --single-instance allow creating sessions with only background processes

kitty/boss.py

@@ -822,6 +822,15 @@ class Boss:
                 args.directory = os.path.join(data['cwd'], args.directory)
             focused_os_window = os_window_id = 0
             for session in create_sessions(opts, args, respect_cwd=True):
+                if not session.has_non_background_processes:
+                    # background only do not create and OS Window
+                    from .launch import LaunchSpec, launch
+                    for tab in session.tabs:
+                        for window in tab.windows:
+                            if window.is_background_process:
+                                assert isinstance(window.launch_spec, LaunchSpec)
+                                launch(get_boss(), window.launch_spec.opts, window.launch_spec.args)
+                    continue
                 os_window_id = self.add_os_window(
                     session, wclass=args.cls, wname=args.name, opts_for_size=opts, startup_id=startup_id,
                     override_title=args.title or None)

kitty/session.py

@@ -39,6 +39,11 @@ class WindowSpec:
     def __init__(self, launch_spec: Union['LaunchSpec', 'SpecialWindowInstance']):
         self.launch_spec = launch_spec
         self.resize_spec: Optional[ResizeSpec] = None
+        self.is_background_process = False
+        if hasattr(launch_spec, 'opts'):  # LaunchSpec
+            from .launch import LaunchSpec
+            assert isinstance(launch_spec, LaunchSpec)
+            self.is_background_process = launch_spec.opts.type == 'background'
 
 
 class Tab:
@@ -53,6 +58,13 @@ class Tab:
         self.cwd: Optional[str] = None
         self.next_title: Optional[str] = None
 
+    @property
+    def has_non_background_processes(self) -> bool:
+        for w in self.windows:
+            if not w.is_background_process:
+                return True
+        return False
+
 
 class Session:
 
@@ -65,6 +77,13 @@ class Session:
         self.os_window_state: Optional[str] = None
         self.focus_os_window: bool = False
 
+    @property
+    def has_non_background_processes(self) -> bool:
+        for t in self.tabs:
+            if t.has_non_background_processes:
+                return True
+        return False
+
     def add_tab(self, opts: Options, name: str = '') -> None:
         if self.tabs and not self.tabs[-1].windows:
             del self.tabs[-1]