How to implement standalone modal routes?

[evertdespiegeleer]

I'm trying to accomplish the following.

I want to add a route to my application (/settings). It's got subroutes, layout logic etc; A proper, serious route like any other.

There is one thing which is special about it:
The uppermost layout component of this route makes it a modal. It's a little silly if the background behind a model disappears when it opens, so I'd like to keep the background in place.

Essentially, I'd like to have two routes rendered at the same time:

• The live route
• The previous route, if one exists

If no previous route exists, nothing should be rendered in the background.

I can come up with all sorts of workarounds but none are great. Ideally, this modal route is an actual, true self-standing route.
A similar discussion exists. It mentions parallel routes, but that as well wouldn't be suitable here. I'm trying to build a modal reachable from anywhere. The route has no relation to the previous page whatsoever.

I can't imagine that this is uncommon, yet I haven't found a solution for it.
I don't think it's possible to keep the previous route from unmounting during navigation, and I don't think it's possible to (re)render the previous route (or any arbitrary route) while actually on another route.