Threading problem when using multiple frames

[keinhaar]

Concurrent access when using iframes. The same error described here for firefox happens with htmlunit-driver: https://bugzilla.mozilla.org/show_bug.cgi?id=1833637 and here https://github.com/gwtproject/gwt/pull/9800

The proposed bugfix for GWT works, but the problem seems to be deeper in the Thread handling of the browser engine, allowing 2 threads to run simultaneously. Chromium based browsers don't show the problem.