Only consider visible render widgets for lost focusProxy

Otherwise, when commenting out the focusProxy way above, and using "foo !npm"
with DuckDuckGo, we get two children (one visible, one invisible).

qutebrowser/browser/webengine/webview.py

@@ -74,7 +74,11 @@ class WebEngineView(QWebEngineView):
 
         # This should only find the RenderWidgetHostViewQtDelegateWidget,
         # but not e.g. a QMenu
-        children = self.findChildren(QQuickWidget)
+        children = [c for c in self.findChildren(QQuickWidget)
+                    if c.isVisible()]
+
+        log.webview.debug("Found possibly lost focusProxy: {}"
+                          .format(children))
 
         if not children:
             return None