There are various test that check that then cannot be intercepted by script.

For example:

• /streams/piping/then-interception.any.js
• /fetch/api/response/response-stream-with-broken-then.any.js

This leads to two potential problems in our code:

1. A crash with assertion failed: incumbent_global.is_some(), as noted at Intermittent CRASH in /fetch/api/response/response-stream-with-broken-then.any.html #35419
2. A failing test due to a succcessful interception.

On the topic of 2 above, I think this was already discussed at #34252. But I think the problem may be different from what was initially discussed, and anyway it is a good idea to centralize the discussion here.