The Popup anchor option is documented as

A string indicating the part of the Popup that should be positioned closest to the coordinate set via Popup#setLngLat . Options are 'center' , 'top' , 'bottom' , 'left' , 'right' , 'top-left' , 'top-right' , 'bottom-left' , and 'bottom-right' . If unset the anchor will be dynamically set to ensure the popup falls within the map container with a preference for 'bottom' .

I'd like to retain the dynamic feature to ensure the popup doesn't fall off the map, while setting a preference other than bottom.

I think the best solution is to change the definition of the anchor option to be the preferred anchor and introduce a new fixedAnchor option to tell if you want it to avoid falling of the map or not.

This is a backwards incompatible change, but I think any change made in a backwards compatible way would introduce to much complexity to the API (for example leaving anchor as is and adding a new anchorPreference option).

I don't mind trying to do a PR, but what do people think about this breaking change?