I got this idea suggestion while watching this video: https://www.youtube.com/watch?v=KcXRjB8N1dE
Somewhere in the first half of the video, Lord Lambert wants to build an artisan quarter in Londonia, but can't find where he can build it. I'm guessing that the available position was in the northern region, which was joined to Londonia's region.

What I'm suggesting is that when the player picks a quarter to build in a city, in addition to all available positions being shown with a "ghost quarter" on the map as currently happens, the city builds UI could have 2 buttons that "scroll" through all available positions. This will make it easier to make sure you don't miss any available position in a city with a large area attached to it.

Boy, are you gonna scroll for long...

Maybe in that case, but in some cases there are only a few valid locations and they might be really far off or hard to notice, so this'll help in those situations