Make sure the config default values are mutable

While the old values meant the same thing, they weren't mutable, so the config
couldn't modify them with a simple .append().

Fixes #3104

qutebrowser/config/configdata.yml

@@ -583,7 +583,7 @@ content.user_stylesheets:
     name: ListOrValue
     valtype: File
     none_ok: True
-  default: null
+  default: []
   desc: A list of user stylesheet filenames to use.
 
 content.webgl:
@@ -1334,7 +1334,7 @@ url.start_pages:
   type:
     name: ListOrValue
     valtype: FuzzyUrl
-  default: "https://start.duckduckgo.com"
+  default: ["https://start.duckduckgo.com"]
   desc: The page(s) to open at the start.
 
 url.yank_ignored_parameters:

tests/unit/config/test_configdata.py

@@ -30,16 +30,28 @@ from qutebrowser.utils import usertypes
 
 
 def test_init(config_stub):
-    """Test reading the default yaml file and validating the values."""
+    """Test reading the default yaml file."""
     # configdata.init() is called by config_stub
     config_stub.val.aliases = {}
     assert isinstance(configdata.DATA, dict)
     assert 'ignore_case' in configdata.DATA
+
+
+def test_data(config_stub):
+    """Test various properties of the default values."""
     for option in configdata.DATA.values():
         # Make sure to_py and to_str work
         option.typ.to_py(option.default)
         option.typ.to_str(option.default)
 
+        # https://github.com/qutebrowser/qutebrowser/issues/3104
+        # For lists/dicts, don't use None as default
+        if isinstance(option.typ, (configtypes.Dict, configtypes.List)):
+            assert option.default is not None
+        # For ListOrValue, use a list as default
+        if isinstance(option.typ, configtypes.ListOrValue):
+            assert isinstance(option.default, list)
+
 
 # https://github.com/qutebrowser/qutebrowser/issues/2777
 @pytest.mark.no_ci

tests/unit/config/test_configfiles.py

@@ -461,6 +461,20 @@ class TestConfigPy:
         assert config.instance._values['aliases']['foo'] == 'message-info foo'
         assert config.instance._values['aliases']['bar'] == 'message-info bar'
 
+    @pytest.mark.parametrize('option, value', [
+        ('content.user_stylesheets', 'style.css'),
+        ('url.start_pages', 'https://www.python.org/'),
+    ])
+    def test_appending(self, config_tmpdir, confpy, option, value):
+        """Test appending an item to some special list types.
+
+        See https://github.com/qutebrowser/qutebrowser/issues/3104
+        """
+        (config_tmpdir / 'style.css').ensure()
+        confpy.write('c.{}.append("{}")'.format(option, value))
+        confpy.read()
+        assert config.instance._values[option][-1] == value
+
     def test_oserror(self, tmpdir, data_tmpdir, config_tmpdir):
         with pytest.raises(configexc.ConfigFileErrors) as excinfo:
             configfiles.read_config_py(str(tmpdir / 'foo'))