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
gwtproject/gwt#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.