In reply to Stephen Vaughan.

Full Site Editing came to WordPress pagebuilders around 2014 with the Headway Theme and also Pagelines partially. In Headway, designers could drag and drop on their evolving page canvas -> headers including menu elements, sidebars+widgets, content blocks, footers any where they wanted so long as the dimensions did not overlap. A Headway Architect edition provided for templates and some layering of elements.
Alas, Headway Architect did not come to pass. But layering of UI page content is alive and well with both modal and non-modal popups and sliders/galleries from a number of vendors [Smart Slider 3, Popup Builder, Elemento to name just a few]
Here is hoping that Gutenberg FSE and Block Theme “core” standards do not suffocate/supplant this lively WordPress UI innovation.