Add content.canvas_reading setting

See #2377
Closes #2235

doc/changelog.asciidoc

@@ -47,6 +47,8 @@ Added
     (requires Qt 5.10).
   * New `content.webrtc_public_interfaces_only` setting to only expose public
     interfaces over WebRTC (requires Qt 5.9.2 or 5.11).
+  * New `content.canvas_reading` setting to disable reading from canvas
+    elements.
 
 Changed
 ~~~~~~~

doc/help/settings.asciidoc

@@ -113,6 +113,7 @@
 |<<content.cache.appcache,content.cache.appcache>>|Enable support for the HTML 5 web application cache feature.
 |<<content.cache.maximum_pages,content.cache.maximum_pages>>|Maximum number of pages to hold in the global memory page cache.
 |<<content.cache.size,content.cache.size>>|Size (in bytes) of the HTTP network cache. Null to use the default value.
+|<<content.canvas_reading,content.canvas_reading>>|Allow websites to read canvas elements.
 |<<content.cookies.accept,content.cookies.accept>>|Which cookies to accept.
 |<<content.cookies.store,content.cookies.store>>|Store cookies.
 |<<content.default_encoding,content.default_encoding>>|Default encoding to use for websites.
@@ -1517,6 +1518,18 @@ Type: <<types,Int>>
 
 Default: empty
 
+[[content.canvas_reading]]
+=== content.canvas_reading
+Allow websites to read canvas elements.
+Note this is needed for some websites to work properly.
+This setting requires a restart.
+
+Type: <<types,Bool>>
+
+Default: +pass:[true]+
+
+This setting is only available with the QtWebEngine backend.
+
 [[content.cookies.accept]]
 === content.cookies.accept
 Which cookies to accept.

qutebrowser/config/configdata.yml

@@ -237,6 +237,16 @@ content.cache.size:
 
     With QtWebEngine, the maximum supported value is 2147483647 (~2 GB).
 
+content.canvas_reading:
+  default: true
+  type: Bool
+  backend: QtWebEngine
+  restart: true
+  desc: >-
+    Allow websites to read canvas elements.
+
+    Note this is needed for some websites to work properly.
+
 # Defaults from QWebSettings::QWebSettings() in
 # qtwebkit/Source/WebKit/qt/Api/qwebsettings.cpp
 

qutebrowser/config/configinit.py

@@ -176,6 +176,9 @@ def qt_args(namespace):
         if config.val.qt.force_software_rendering == 'chromium':
             argv.append('--disable-gpu')
 
+        if not config.val.content.canvas_reading:
+            argv.append('--disable-reading-from-canvas')
+
         if not qtutils.version_check('5.11'):
             # On Qt 5.11, we can control this via QWebEngineSettings
             if not config.val.content.autoplay:

tests/unit/config/test_configinit.py

@@ -451,6 +451,20 @@ class TestQtArgs:
         arg = '--force-webrtc-ip-handling-policy=default_public_interface_only'
         assert (arg in args) == added
 
+    @pytest.mark.parametrize('backend, canvas_reading, added', [
+        (usertypes.Backend.QtWebEngine, True, False),  # enabled
+        (usertypes.Backend.QtWebKit, False, False),  # QtWebKit
+        (usertypes.Backend.QtWebEngine, False, True),
+    ])
+    def test_canvas_reading(self, config_stub, monkeypatch, parser,
+                            backend, canvas_reading, added):
+        config_stub.val.content.canvas_reading = canvas_reading
+        monkeypatch.setattr(configinit.objects, 'backend', backend)
+
+        parsed = parser.parse_args([])
+        args = configinit.qt_args(parsed)
+        assert ('--disable-reading-from-canvas' in args) == added
+
 
 @pytest.mark.parametrize('arg, confval, used', [
     # overridden by commandline arg