We'd need to see the code generated by the editor to create the container for the forum, but it's likely to be fixed in height in some way.
This is really an issue for the support team of the editor, not Nabble.
It might be possible to help if post the URL of your site so the code can be seen.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.