Defer Block + HMR automatically prefeches chunks

[JeanMeche]

Description

When HMR is enabled, defered chunks are automatically prefetched despite any prefetching triggers.

repro: https://github.com/JeanMeche/test-defer.git

[thePunderWoman]

This is expected and unavoidable due to the nature of HMR. The initial load should behave as normal, but after changes, the modules are already fetched / cached and the page hasn't refreshed. We really just need to update the defer guide to make this clear. I think we need this as a callout both on the @defer guide and the incremental hydration guide.

[JeanMeche]

What I noticed it that the defered component is pre-fetched on bootstrap.

[thePunderWoman]

Per our offline conversation, it's not prefetched. It's bundled in with the main bundle with HMR enabled. This is probably expected due to HMR complexity, but definitely confusing. It needs follow up.