The idea here is to display all block controls while editing content, but apply some kind of “disabled” visual treatment to them. Adjusting the opacity is probably easiest, but there are most likely other options to explore as this may not be accessible enough (color contrast). I’m keen to get the flow right initially and focus on these finer details later.
You can navigate directly to the template via the ellipsis menu on the block.
Alternatively, clicking any “disabled” control will open a dialog asking the user if they’d like to edit the template, and briefly reminding them of the implications of doing so.
The transition to template editing view is made clear via the UI changes proposed in #27849 (IE the inverted top bar). Let’s not dwell on that in this issue.
In template editing view the previously “disabled” controls are now enabled and you can make any changes you need to. However, the contents of the block (“Hello World!”) is locked since we are now editing the template. To get back to editing the content you can click the “← Return to post” link, or simply double click on the text.
Any changes to either the post or the template will be surfaced in the multi-entity save flow.
More discussion here.