Add a setting for Chromium's low-end device mode

See #4039
See #2377

doc/changelog.asciidoc

@@ -34,6 +34,8 @@ Added
   * Opening PDFs on https:// sites now works properly.
 - New `qt.process_model` setting which can be used to change Chromium's process
   model.
+- New `qt.low_end_device_mode` setting which turns on Chromium's low-end device
+  mode. This mode uses less RAM, but the expense of performance.
 
 Changed
 ~~~~~~~

doc/help/settings.asciidoc

@@ -229,6 +229,7 @@
 |<<qt.force_platform,qt.force_platform>>|Force a Qt platform to use.
 |<<qt.force_software_rendering,qt.force_software_rendering>>|Force software rendering for QtWebEngine.
 |<<qt.highdpi,qt.highdpi>>|Turn on Qt HighDPI scaling.
+|<<qt.low_end_device_mode,qt.low_end_device_mode>>|Use Chromium's low-end device mode.
 |<<qt.process_model,qt.process_model>>|Which Chromium process model to use.
 |<<scrolling.bar,scrolling.bar>>|Show a scrollbar.
 |<<scrolling.smooth,scrolling.smooth>>|Enable smooth scrolling for web pages.
@@ -2758,6 +2759,24 @@ Type: <<types,Bool>>
 
 Default: +pass:[false]+
 
+[[qt.low_end_device_mode]]
+=== qt.low_end_device_mode
+Use Chromium's low-end device mode.
+This improves the RAM usage of renderer processes, at the expense of performance.
+This setting requires a restart.
+
+Type: <<types,String>>
+
+Valid values:
+
+ * +force-on+: Always use low-end device mode.
+ * +auto+: Decide automatically (uses low-end mode with < 1 GB available RAM).
+ * +force-off+: Never use low-end device mode.
+
+Default: +pass:[auto]+
+
+This setting is only available with the QtWebEngine backend.
+
 [[qt.process_model]]
 === qt.process_model
 Which Chromium process model to use.

qutebrowser/config/configdata.yml

@@ -210,6 +210,23 @@ qt.process_model:
       - https://www.chromium.org/developers/design-documents/process-models
       - https://doc.qt.io/qt-5/qtwebengine-features.html#process-models
 
+qt.low_end_device_mode:
+  type:
+    name: String
+    valid_values:
+      - force-on: Always use low-end device mode.
+      - auto: Decide automatically (uses low-end mode with < 1 GB available
+            RAM).
+      - force-off: Never use low-end device mode.
+  default: auto
+  backend: QtWebEngine
+  restart: true
+  desc: >-
+    Use Chromium's low-end device mode.
+
+    This improves the RAM usage of renderer processes, at the expense of
+    performance.
+
 qt.highdpi:
   type: Bool
   default: false

qutebrowser/config/configinit.py

@@ -202,4 +202,15 @@ def qt_args(namespace):
             raise utils.Unreachable("Unknown process model {}"
                                     .format(process_model))
 
+        low_end_device_mode = config.val.qt.low_end_device_mode
+        if low_end_device_mode == 'auto':
+            pass
+        elif low_end_device_mode == 'force-on':
+            argv.append('--enable-low-end-device-mode')
+        elif low_end_device_mode == 'force-off':
+            argv.append('--disable-low-end-device-mode')
+        else:
+            raise utils.Unreachable("Unknown low-end device mode {}"
+                                    .format(low_end_device_mode))
+
     return argv

tests/unit/config/test_configinit.py

@@ -492,6 +492,26 @@ class TestQtArgs:
             assert '--process-per-site-instance' not in args
             assert '--process-per-tab' not in args
 
+    @pytest.mark.parametrize('low_end_device_mode, arg', [
+        ('auto', None),
+        ('force-on', '--enable-low-end-device-mode'),
+        ('force-off', '--disable-low-end-device-mode'),
+    ])
+    def test_low_end_device_mode(self, config_stub, monkeypatch, parser,
+                                 low_end_device_mode, arg):
+        monkeypatch.setattr(configinit.objects, 'backend',
+                            usertypes.Backend.QtWebEngine)
+
+        config_stub.val.qt.low_end_device_mode = low_end_device_mode
+        parsed = parser.parse_args([])
+        args = configinit.qt_args(parsed)
+
+        if arg is None:
+            assert '--enable-low-end-device-mode' not in args
+            assert '--disable-low-end-device-mode' not in args
+        else:
+            assert arg in args
+
 
 @pytest.mark.parametrize('arg, confval, used', [
     # overridden by commandline arg