Use the style override only for macOS

We currently only need it there. This prevents unwanted side effects on other platforms.

picard/tagger.py

@@ -192,7 +192,6 @@ class Tagger(QtWidgets.QApplication):
         self.__class__.__instance = self
         setup_config(self, picard_args.config_file)
         config = get_config()
-        self.setStyle(OverrideStyle())
         theme.setup(self)
 
         self._cmdline_files = picard_args.FILE
@@ -1051,19 +1050,6 @@ def process_picard_args():
     return parser.parse_known_args()[0]
 
 
-class OverrideStyle(QtWidgets.QProxyStyle):
-    """Override the default style to fix some platform specific issues"""
-
-    def styleHint(self, hint, option, widget, returnData):
-        # This is disabled on macOS, but prevents collapsing tree view items easily with
-        # left arrow key. Enable this consistently on all platforms.
-        # See https://tickets.metabrainz.org/browse/PICARD-2417
-        # and https://bugreports.qt.io/browse/QTBUG-100305
-        if hint == QtWidgets.QStyle.StyleHint.SH_ItemView_ArrowKeysNavigateIntoChildren:
-            return True
-        return super().styleHint(hint, option, widget, returnData)
-
-
 def main(localedir=None, autoupdate=True):
     # Some libs (ie. Phonon) require those to be set
     QtWidgets.QApplication.setApplicationName(PICARD_APP_NAME)

picard/ui/theme.py

@@ -27,6 +27,7 @@ from enum import Enum
 from PyQt5 import (
     QtCore,
     QtGui,
+    QtWidgets,
 )
 
 from picard import log
@@ -91,6 +92,19 @@ dark_syntax_theme = SyntaxTheme(
 )
 
 
+class MacOverrideStyle(QtWidgets.QProxyStyle):
+    """Override the default style to fix some platform specific issues"""
+
+    def styleHint(self, hint, option, widget, returnData):
+        # This is disabled on macOS, but prevents collapsing tree view items easily with
+        # left arrow key. Enable this consistently on all platforms.
+        # See https://tickets.metabrainz.org/browse/PICARD-2417
+        # and https://bugreports.qt.io/browse/QTBUG-100305
+        if hint == QtWidgets.QStyle.StyleHint.SH_ItemView_ArrowKeysNavigateIntoChildren:
+            return True
+        return super().styleHint(hint, option, widget, returnData)
+
+
 class BaseTheme:
     def __init__(self):
         self._dark_theme = False
@@ -104,6 +118,8 @@ class BaseTheme:
         # across all OSes.
         if not IS_MACOS and not IS_HAIKU and self._loaded_config_theme != UiTheme.SYSTEM:
             app.setStyle('Fusion')
+        elif IS_MACOS:
+            app.setStyle(MacOverrideStyle(app.style()))
 
         app.setStyleSheet(
             'QGroupBox::title { /* PICARD-1206, Qt bug workaround */ }'