Add a test for the system monospace font

See #5663

qutebrowser/config/configutils.py

@@ -282,6 +282,9 @@ def __init__(self, families: Sequence[str]) -> None:
     def __iter__(self) -> Iterator[str]:
         yield from self._families
 
+    def __len__(self) -> int:
+        return len(self._families)
+
     def __repr__(self) -> str:
         return utils.get_repr(self, families=self._families, constructor=True)
 

tests/unit/config/test_configutils.py

@@ -21,6 +21,7 @@
 from hypothesis import strategies
 import pytest
 from PyQt5.QtCore import QUrl
+from PyQt5.QtWidgets import QLabel
 
 from qutebrowser.config import configutils, configdata, configtypes, configexc
 from qutebrowser.utils import urlmatch, usertypes, qtutils
@@ -364,3 +365,26 @@ def test_from_str_hypothesis(self, family_str):
             assert family
 
         str(families)
+
+    def test_system_default_basics(self, qapp):
+        families = configutils.FontFamilies.from_system_default()
+        assert len(families) == 1
+        assert str(families)
+
+    def test_system_default_rendering(self, qtbot):
+        families = configutils.FontFamilies.from_system_default()
+
+        label = QLabel()
+        qtbot.add_widget(label)
+        label.setText("Hello World")
+
+        stylesheet = f'font-family: {families.to_str(quote=True)}'
+        print(stylesheet)
+        label.setStyleSheet(stylesheet)
+
+        with qtbot.waitExposed(label):
+            # Needed so the font gets calculated
+            label.show()
+
+        info = label.fontInfo()
+        assert info.fixedPitch(), info.family()