The existing flow from editing a post to editing the post template creates a view where the user is editing a template with a specific entity loaded in to provide context.

In the site editor it is also currently possible to edit a template without any context, and see placeholder content in lieu of actual post data.

Since all the pieces exist to do so, we should consider allowing the user to switch which entity is being previewed while editing a template. This will be a useful tool to ensure that any template edits in the current session will play nicely with a variety of compatible entires.

We can utilise the "View" menu (proposed in #27847) to contain this affordance.

Because the organisation of content can get quite complicated on larger sites, we might use the drilldown pattern from the site editor navigation to keep this menu simple:

It is (hopefully) quite easy to imagine different sections for different content types.

Once content is selected, an interstitial indicates which blocks are looking up post data.