Consider a "same page preview" feature for content editing in core

[klonos]

I was made aware of https://www.drupal.org/project/same_page_preview during a session in the recent Backdrop LIVE event, and there's an issue to get that functionality into Drupal core: https://www.drupal.org/project/ideas/issues/3367356

The project page in d.o has a short, ~3min video that demonstrates what the module does: https://youtu.be/Mh_plCpt1_A

I was thinking that since we already have a way to show a "live preview" in the theme color settings, then perhaps portions of that could be reused to implement something similar for this feature here.

Not sure about the feasibility, challenges, effort required, or whether that should be in core or not. Starting this issue here to serve as a to-do to loop back into and for discussion to start.