Best strategy to create a foreground mask in Map Viewer
To get around the popups, I would try to apply attribute-based filters to your layers if possible. I see you do have some on already, but more of the layers include a county attribute:
Once that's applied to all possible layers (most importantly the Open Space areas), the map is already starting to look more spatially focused, even without the Foreground layer.
The big issue now are the transportation lines. But are popups strictly necessary for those? The way they're currently configured doesn't seem to serve much purpose.
If you simply disable popups on the features that can't be filtered, you'll have a solid, location-specific map, and clicking outside the county boundaries won't show anything.
Final note: you might consider publishing that foreground layer as a vector tile, as that will scale nicely, and won't have to re-load as you pan around the map, resulting in layers "peeking" through the foreground as you pan.
