Templates: Focus mode: Constrain the view to the size of the template part

[scruffian]

What problem does this address?

When editing template parts in focus mode, its hard to understand how they fit into the broader context of the site because there is so much white space:

What is your proposed solution?

Ideally the iframe would display at the same size as the template is rendered at in the site, something like this:

[Mamaduka]

Duplicate of #42226.