I downloaded "Postgres.app with all currently supported PostgreSQL versions" v2.3.5, with a view to running against a 9.6 server version. I see absolutely no option to select any server version other than 12 and I see no documentation at all about selecting which server version to run against. What am I missing here?
(comment was edited)
Show the drawer using this button:

Click the + to create a new server

then you should have different versions in this dropdown:

Thanks very much for taking the the time for me @cappert....I thought I was going insane there for a while! 馃槃
So directly after installation, I had to find the reveal side-bar button in order to find the plus button. It might be an idea to just have the sidebar open by default? It's not exactly a hugely complex screen with space at an absolute premium.
We discussed this several times before, the idea is to keep the UI as simple as possible for the majority of users only interested in running a single instance. But as this issue has been confusing people in the past several times (see #388, #422, #429, #484, #523) we should finally improve this. There ideas for this discussed in the open cases https://github.com/PostgresApp/PostgresApp/issues/445#issuecomment-385428585 and https://github.com/PostgresApp/PostgresApp/issues/399#issuecomment-316660460
Beside improvements to documentation, esp. on upgrades, my proposal for the App's behaviour would be:
(*): we currently can only detect the first run in total, not per release, IIRC. At this, potentially existing data dirs are scanned and server-entries for them created.
Does this sound useful or is it to complicated? The simpler proposal from https://github.com/PostgresApp/PostgresApp/issues/399#issuecomment-316660460 I would read as:
To me this defeats the concept of an expandable sidebar in general and may annoy people intentionally running an older version.
I think you need to start tracking and surfacing the versions in the UI, without falling into the slippery slope of upgrading users' data for them, which I think is too difficult and error prone.
I made a very quick mockup.

Some thoughts unrelated to the mockup:
I think the closed drawer pattern is too "old-school" and most apps start with a sidebar launch with it open by default (from the top of my head- Slack, VSCode, Garageband, Whatsapp), following web UI trends. If there's only one server it's just empty space.
There's the risk of confusion between creative a server and creating a database though.
One other problem is that the error message "The binaries for the PostgreSQL server were not found" is too generic and cryptic.
Maybe you need to show somewhere what are the capabilities of the installed Postgres.app, now that you have different flavors (12, 12+11+10, etc.). I feel it's a big source of confusion.
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(comment was edited)
Show the drawer using this button:

Click the + to create a new server

then you should have different versions in this dropdown:
