Automated tests will ensure that there are no regressions. To test that the functionality works, you can deliberately break it: Ensure that behat is initialised Open lib/editor/atto/tests/behat/clean.feature in your preferred editor. In the first Scenario, after the "I click on "When I open my profile in edit mode" step, add: And I pause Run the first scenario only, for example: ./vendor/bin/behat --config=path/to/config.yml `pwd`/lib/editor/atto/tests/behat/clean.feature:6 When the step pauses, take a look at the browser running Behat, and: Confirm that the editor in use is Atto Ctrl+C the behat run Back in your editor Open lib/editor/atto/tests/behat/behat_editor_atto.php Find the set_default_editor_flag() function} Change the last line fo the function to set the default editor to 'tinymce' Re-run the test When the step pauses, take a look at the browser running Behat, and: Confirm that the editor in use has changed to the legacy TinyMCE editor Ctrl+C the behat run Open lib/editor/atto/plugins/bold/tests/behat/bold.feature Remove the @editor_atto tag entirely Add the same "And I pause" step Run that scenario from the bold.feature Confirm that once again the editor was set to TinyMCE Undo the changes to lib/editor/atto/tests/behat/behat_editor_atto.php Re-run the test When the step pauses, take a look at the browser running Behat, and Confirm that the editor in use has changed back to the Atto editor