tests: Fix JS header tests

This is a follow-up to f0eed465ab
See #4657

pytest.ini

@@ -30,6 +30,7 @@ markers =
     fake_os: Fake utils.is_* to a fake operating system
     unicode_locale: Tests which need an unicode locale to work
     qtwebkit6021_xfail: Tests which would fail on WebKit version 602.1
+    js_headers: Sets JS headers dynamically on QtWebEngine (unsupported on some versions)
 qt_log_level_fail = WARNING
 qt_log_ignore =
     ^SpellCheck: .*

tests/end2end/conftest.py

@@ -137,6 +137,11 @@ if not getattr(sys, 'frozen', False):
 
 def pytest_collection_modifyitems(config, items):
     """Apply @qtwebengine_* markers; skip unittests with QUTE_BDD_WEBENGINE."""
+    # WORKAROUND for https://bugreports.qt.io/browse/QTBUG-75884
+    # (note this isn't actually fixed properly before Qt 5.15)
+    header_bug_fixed = (not qtutils.version_check('5.12', compiled=False) or
+                        qtutils.version_check('5.15', compiled=False))
+
     markers = [
         ('qtwebengine_todo', 'QtWebEngine TODO', pytest.mark.xfail,
          config.webengine),
@@ -152,6 +157,9 @@ def pytest_collection_modifyitems(config, items):
          config.webengine),
         ('qtwebengine_mac_xfail', 'Fails on macOS with QtWebEngine',
          pytest.mark.xfail, config.webengine and utils.is_mac),
+        ('js_headers', 'Sets headers dynamically via JS',
+         pytest.mark.skipif,
+         config.webengine and not header_bug_fixed),
     ]
 
     for item in items:

tests/end2end/features/misc.feature

@@ -349,18 +349,24 @@ Feature: Various utility commands.
 
     # This still doesn't set window.navigator.language
     # See https://bugreports.qt.io/browse/QTBUG-61949
-    @qtwebkit_skip
+    @qtwebkit_skip @js_headers
     Scenario: Accept-Language header (JS)
         When I set content.headers.accept_language to it,fr
         And I run :jseval console.log(window.navigator.languages)
         Then the javascript message "it,fr" should be logged
 
-    Scenario: Setting a custom user-agent header
+    Scenario: User-agent header
         When I set content.headers.user_agent to toaster
         And I open headers
         And I run :jseval console.log(window.navigator.userAgent)
         Then the header User-Agent should be set to toaster
-        And the javascript message "toaster" should be logged
+
+    @js_headers
+    Scenario: User-agent header (JS)
+        When I set content.headers.user_agent to toaster
+        And I open about:blank
+        And I run :jseval console.log(window.navigator.userAgent)
+        Then the javascript message "toaster" should be logged
 
     ## https://github.com/qutebrowser/qutebrowser/issues/1523