diff --git a/dangerzone/gui/__init__.py b/dangerzone/gui/__init__.py
index ac5736e..0d0f899 100644
--- a/dangerzone/gui/__init__.py
+++ b/dangerzone/gui/__init__.py
@@ -6,41 +6,14 @@ from typing import Optional
 
 import click
 import uuid
-from PySide6 import QtCore
-from PySide6.QtCore import QEvent
-from PySide6.QtWidgets import QApplication
 
+from .application import Application
 from .common import GuiCommon
 from .main_window import MainWindow
 from .systray import SysTray
 from ..global_common import GlobalCommon
 
 
-class Application(QApplication):
-    document_selected = QtCore.Signal(str)
-    new_window = QtCore.Signal()
-    application_activated = QtCore.Signal()
-
-    def __init__(self):
-        super(Application, self).__init__()
-        self.setQuitOnLastWindowClosed(False)
-        self.original_event = self.event
-
-        def monkeypatch_event(event: QEvent):
-            # In macOS, handle the file open event
-            if event.type() == QtCore.QEvent.FileOpen:
-                # Skip file open events in dev mode
-                if not hasattr(sys, "dangerzone_dev"):
-                    self.document_selected.emit(event.file())
-                    return True
-            elif event.type() == QtCore.QEvent.ApplicationActivate:
-                self.application_activated.emit()
-                return True
-            return self.original_event(event)
-
-        self.event = monkeypatch_event
-
-
 @click.command()
 @click.argument("filename", required=False)
 def gui_main(filename):
@@ -50,7 +23,7 @@ def gui_main(filename):
 
         # Strip ANSI colors from stdout output, to prevent terminal colors from breaking
         # the macOS GUI app
-        from strip_ansi import strip_ansi
+        from strip_ansi import strip_ansi  # type: ignore
 
         class StdoutFilter:
             def __init__(self, stream):
@@ -95,7 +68,7 @@ def gui_main(filename):
             len(windows) == 1
             and windows[list(windows.keys())[0]].common.input_filename is None
         ):
-            window = windows[list(windows.keys())[0]]
+            window: MainWindow = windows[list(windows.keys())[0]]
         else:
             window_id = uuid.uuid4().hex
             window = MainWindow(global_common, gui_common, window_id)
diff --git a/dangerzone/gui/application.py b/dangerzone/gui/application.py
new file mode 100644
index 0000000..565ae2e
--- /dev/null
+++ b/dangerzone/gui/application.py
@@ -0,0 +1,30 @@
+import sys
+
+from PySide6 import QtCore
+from PySide6.QtCore import QEvent
+from PySide6.QtWidgets import QApplication
+
+
+class Application(QApplication):
+    document_selected = QtCore.Signal(str)
+    new_window = QtCore.Signal()
+    application_activated = QtCore.Signal()
+
+    def __init__(self):
+        super(Application, self).__init__()
+        self.setQuitOnLastWindowClosed(False)
+        self.original_event = self.event
+
+        def monkeypatch_event(event: QEvent):
+            # In macOS, handle the file open event
+            if event.type() == QtCore.QEvent.FileOpen:
+                # Skip file open events in dev mode
+                if not hasattr(sys, "dangerzone_dev"):
+                    self.document_selected.emit(event.file())
+                    return True
+            elif event.type() == QtCore.QEvent.ApplicationActivate:
+                self.application_activated.emit()
+                return True
+            return self.original_event(event)
+
+        self.event = monkeypatch_event
diff --git a/dangerzone/gui/common.py b/dangerzone/gui/common.py
index 9fc5f84..7628adf 100644
--- a/dangerzone/gui/common.py
+++ b/dangerzone/gui/common.py
@@ -6,13 +6,16 @@ import pipes
 from PySide6 import QtCore, QtGui, QtWidgets
 from colorama import Fore
 
+from . import Application
+from ..global_common import GlobalCommon
+
 if platform.system() == "Darwin":
     import plistlib
 
 elif platform.system() == "Linux":
     import grp
     import getpass
-    from xdg.DesktopEntry import DesktopEntry
+    from xdg.DesktopEntry import DesktopEntry  # type: ignore
 
 from ..settings import Settings
 
@@ -22,7 +25,7 @@ class GuiCommon(object):
     The GuiCommon class is a singleton of shared functionality for the GUI
     """
 
-    def __init__(self, app, global_common):
+    def __init__(self, app: Application, global_common: GlobalCommon):
         # Qt app
         self.app = app
 
@@ -45,7 +48,7 @@ class GuiCommon(object):
             path = self.global_common.get_resource_path("icon.png")
         return QtGui.QIcon(path)
 
-    def open_pdf_viewer(self, filename):
+    def open_pdf_viewer(self, filename: str):
         if platform.system() == "Darwin":
             # Open in Preview
             args = ["open", "-a", "Preview.app", filename]
@@ -75,7 +78,8 @@ class GuiCommon(object):
             print(Fore.YELLOW + "> " + Fore.CYAN + args_str)
             subprocess.Popen(args)
 
-    def _find_pdf_viewers(self):
+    @staticmethod
+    def _find_pdf_viewers():
         pdf_viewers = {}
         if platform.system() == "Linux":
             # Find all .desktop files
diff --git a/dangerzone/gui/main_window.py b/dangerzone/gui/main_window.py
index c0893b2..56ca217 100644
--- a/dangerzone/gui/main_window.py
+++ b/dangerzone/gui/main_window.py
@@ -7,14 +7,16 @@ import shutil
 from PySide6 import QtCore, QtGui, QtWidgets
 from colorama import Style, Fore
 
+from . import GuiCommon
 from ..common import Common
 from ..container import convert
+from ..global_common import GlobalCommon
 
 
 class MainWindow(QtWidgets.QMainWindow):
     delete_window = QtCore.Signal(str)
 
-    def __init__(self, global_common, gui_common, window_id):
+    def __init__(self, global_common: GlobalCommon, gui_common: GuiCommon, window_id: str):
         super(MainWindow, self).__init__()
         self.global_common = global_common
         self.gui_common = gui_common
@@ -90,7 +92,7 @@ class MainWindow(QtWidgets.QMainWindow):
 class InstallContainerThread(QtCore.QThread):
     finished = QtCore.Signal()
 
-    def __init__(self, global_common):
+    def __init__(self, global_common: GlobalCommon):
         super(InstallContainerThread, self).__init__()
         self.global_common = global_common
 
@@ -111,7 +113,7 @@ class WaitingWidget(QtWidgets.QWidget):
     # - "install_container"
     finished = QtCore.Signal()
 
-    def __init__(self, global_common, gui_common):
+    def __init__(self, global_common: GlobalCommon, gui_common: GuiCommon):
         super(WaitingWidget, self).__init__()
         self.global_common = global_common
         self.gui_common = gui_common
@@ -145,7 +147,7 @@ class WaitingWidget(QtWidgets.QWidget):
         self.check_state()
 
     def check_state(self):
-        state = None
+        state: str
 
         # Can we find the container runtime binary binary
         if platform.system() == "Linux":
@@ -200,7 +202,7 @@ class WaitingWidget(QtWidgets.QWidget):
 class ContentWidget(QtWidgets.QWidget):
     close_window = QtCore.Signal()
 
-    def __init__(self, global_common, gui_common, common):
+    def __init__(self, global_common: GlobalCommon, gui_common: GuiCommon, common: Common):
         super(ContentWidget, self).__init__()
 
         self.global_common = global_common
@@ -255,7 +257,7 @@ class ContentWidget(QtWidgets.QWidget):
 class DocSelectionWidget(QtWidgets.QWidget):
     document_selected = QtCore.Signal()
 
-    def __init__(self, common):
+    def __init__(self, common: Common):
         super(DocSelectionWidget, self).__init__()
         self.common = common
 
@@ -298,7 +300,7 @@ class SettingsWidget(QtWidgets.QWidget):
     start_clicked = QtCore.Signal()
     close_window = QtCore.Signal()
 
-    def __init__(self, global_common, gui_common, common):
+    def __init__(self, global_common: GlobalCommon, gui_common: GuiCommon, common: Common):
         super(SettingsWidget, self).__init__()
         self.global_common = global_common
         self.gui_common = gui_common
@@ -489,7 +491,7 @@ class ConvertThread(QtCore.QThread):
     is_finished = QtCore.Signal(bool)
     update = QtCore.Signal(bool, str, int)
 
-    def __init__(self, global_common, common):
+    def __init__(self, global_common: GlobalCommon, common: Common):
         super(ConvertThread, self).__init__()
         self.global_common = global_common
         self.common = common
@@ -536,7 +538,7 @@ class ConvertThread(QtCore.QThread):
 class ConvertWidget(QtWidgets.QWidget):
     close_window = QtCore.Signal()
 
-    def __init__(self, global_common, gui_common, common):
+    def __init__(self, global_common: GlobalCommon, gui_common: GuiCommon, common: Common):
         super(ConvertWidget, self).__init__()
         self.global_common = global_common
         self.gui_common = gui_common
diff --git a/dangerzone/gui/systray.py b/dangerzone/gui/systray.py
index ff962f2..080b4ef 100644
--- a/dangerzone/gui/systray.py
+++ b/dangerzone/gui/systray.py
@@ -1,12 +1,12 @@
 from PySide6 import QtWidgets
 
 from dangerzone.global_common import GlobalCommon
-from dangerzone.gui import GuiCommon
+from dangerzone.gui import GuiCommon, Application
 
 
 class SysTray(QtWidgets.QSystemTrayIcon):
     def __init__(
-        self, global_common: GlobalCommon, gui_common: GuiCommon, app
+        self, global_common: GlobalCommon, gui_common: GuiCommon, app: Application
     ):
         super(SysTray, self).__init__()
         self.global_common = global_common