Work around broken error pages on Debian

See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=882805
See qutebrowser#5078, qutebrowser#3773

doc/changelog.asciidoc

@@ -52,6 +52,7 @@ Fixed
 - Downloads are now hidden properly when the browser is in fullscreen mode.
 - Crash when setting `colors.webpage.bg` to an empty value with QtWebKit.
 - Crash when the history database file is not a proper sqlite database.
+- Workaround for missing/broken error pages on Debian.
 
 v1.8.1 (2019-09-27)
 -------------------

qutebrowser/browser/webengine/webenginetab.py

@@ -1357,13 +1357,22 @@ def _on_render_process_terminated(self, status, exitcode):
     def _error_page_workaround(self, html):
         """Check if we're displaying a Chromium error page.
 
-        This gets only called if we got loadFinished(False) without JavaScript,
-        so we can display at least some error page.
+        This gets called if we got a loadFinished(False), so we can display at
+        least some error page in situations where Chromium's can't be
+        displayed.
 
         WORKAROUND for https://bugreports.qt.io/browse/QTBUG-66643
+        WORKAROUND for https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=882805
+
         Needs to check the page content as a WORKAROUND for
         https://bugreports.qt.io/browse/QTBUG-66661
         """
+        js_enabled = self.settings.test_attribute('content.javascript.enabled')
+        missing_jst = 'jstProcess(' in html and 'jstProcess=' not in html
+
+        if js_enabled and not missing_jst:
+            return
+
         match = re.search(r'"errorCode":"([^"]*)"', html)
         if match is None:
             return
@@ -1393,8 +1402,7 @@ def _on_load_finished(self, ok: bool) -> None:
         else:
             self._update_load_status(ok)
 
-        js_enabled = self.settings.test_attribute('content.javascript.enabled')
-        if not ok and not js_enabled:
+        if not ok:
             self.dump_async(self._error_page_workaround)
 
         if ok and self._reload_url is not None: