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Last tested: Dec 24, 2019
By default, embed looks and dashboards will always show the production version, whether or not you are viewing them in production mode. However, if you are currently in development mode, you can add
?force_production=false to the URL for the embed look or dashboard to see it in development mode. This option can be useful for testing changes in the embed context before pushing them to production.
However, note that since this option is not documented, it is not guaranteed that it won't be removed in the future.