Add a qt.highdpi setting

See #1585

qutebrowser/config/configdata.yml

@@ -157,6 +157,19 @@ qt.force_platform:
     using the XCB plugin when running QtWebEngine on Wayland.
 
 
+qt.highdpi:
+  type: Bool
+  default: false
+  desc: >-
+    Turn on Qt HighDPI scaling.
+
+    This is equivalent to setting QT_AUTO_SCREEN_SCALE_FACTOR=1 in the
+    environment.
+
+    It's off by default as it can cause issues with some bitmap fonts.
+    As an alternative to this, it's possible to set font sizes and the
+    `zoom.default` setting.
+
 ## auto_save
 
 auto_save.interval:

qutebrowser/config/configinit.py

@@ -94,6 +94,9 @@ def _init_envvars():
     else:
         os.environ.pop('QT_WAYLAND_DISABLE_WINDOWDECORATION', None)
 
+    if config.val.qt.highdpi:
+        os.environ['QT_AUTO_SCREEN_SCALE_FACTOR'] = '1'
+
 
 @config.change_filter('fonts.monospace', function=True)
 def _update_monospace_fonts():

tests/unit/config/test_configinit.py

@@ -288,6 +288,15 @@ class TestEarlyInit:
         configinit._init_envvars()
         assert os.environ[envvar] == 'toaster'
 
+    def test_highdpi(self, monkeypatch, config_stub):
+        envvar = 'QT_AUTO_SCREEN_SCALE_FACTOR'
+        monkeypatch.delenv(envvar, raising=False)
+
+        config_stub.val.qt.highdpi = True
+
+        configinit._init_envvars()
+        assert os.environ[envvar] == '1'
+
     @pytest.mark.parametrize('old', ['1', '0', None])
     @pytest.mark.parametrize('configval', [True, False])
     def test_hide_wayland_decoration(self, monkeypatch, config_stub,