Some extensions, like Blackout, intentionally interfere with standard operation of MediaWiki and cause browser tests to fail. Extensions should have some way to signal to quibble that browser tests should be skipped for that repository.
Examples:
| AddThis | Injects `share\n` in content
| AccessControl | Denies access by default
| Blackout | Intentionally uses a black overlay over the content
| CommentBox | Inject content in article
| ConfirmAccount | webdriver.io: miss #wpName2
| EditSubpages | Require login T196436
| Premoderation | T157417 Trigger an error to notify edit has been put in a moderation Queue
| UnCaptcha | "I am not a robot" check box on the create account page
| Wikispeech | <div id="ext-wikispeech-control-panel"> and alert() on edit
See also T196436 which suggests to add a wdio test in extension to potentially tune some permissions. But that might be the wrong approach.