Steps to replicate the issue (include links if applicable):

• Enable DevelopmentSettings.php
• Enable Spercial:RestSandbox by configuring $wgRestSandboxSpecs.
• Visite Special:RestSandbox and select an API to explore

What happens?:

• A list of the API's endpoints is shown.
• Clicking on the endpoint (or the arrow icon associated with it) does nothing.
• Clicking on the schemas (or the arrow icon associated with it) does nothing.
• Clicking on the section heading (or the arrow icon associated with it) does nothing either.
• The JavaScript console does not show errors.

What should have happened instead?:

• Clicking on the endpoint (or the arrow icon associated with it) should un-collapse it, revealing details about the endpoint and tools to interact with it.
• Clicking on the section heading (or the arrow icon associated with it) should collapse it.

Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia): 1.43alpha at commit 7c4d6f627b5050e10287752533b31219e0627b8f

Other information (browser name/version, screenshots, etc.):
For context see T362006: Provide a Swagger-UI for exploring the core REST API