While analysing the problems I came across with Gutenberg 9.4.0 I developed an extension to template-part.php which added debug information visible in the front end.
It sandwiched the output for each template part in some debug <div class="WP_DEBUG"> tags
which contained the name of the template part.

I then wrote some naff CSS to enable the information to be viewed on the front end.

Requirements

• Formalise this solution for template parts
• Find a method to automatically produce front end information regarding the template file that's been selected
• Consider ways to easily turn the debug information on and off. e.g. Additional constant ( FSE_DEBUG), logged in admin user.