Convert a Point (or PointLike) to LngLat object

I have a mapbox map, and I want to add a fixed marker. With "fixed" I mean something that is in absolute so fixed respect of the map div container.

Example:

(0,0)         (200,0)
+-------------+
|             |
|             |
|             |
|      *      |
|             |
|             |
|             |
+-------------+
(0,300)       (200, 300)

In that case the marker * is at the center of the map so to place it I can use:

.marker {
  position: absolute;
  top: 50%;
  left: 50%
}

and then use getCenter():

const lngLatObj = map.getCenter()

but if I want the marker not in the center but for example at 20% left and 90% top?

(0,0)         (200,0)
+-------------+
|             |
|             |
|             |
|             |
|             |
|             |
| *           |
+-------------+
(0,300)       (200, 300)

Here you can see the map container and the marker * on that map.

suppose I have the pixel coordinates of *, how can I find the longitude and latitude of that point?

I imagine something like:

const x = 40
const y = 270
const point = new mapboxgl.Point(x, y)
const lngLat = point.??

I didn't have mouse or touch events. The point is fixed but user can zoom and pan the map.

With marker I mean not a mapbox Marker but a simple div element.

Thanks very much

here a reproducible example

Solution

I fix using geodesy:

const boundsNorth = new LatLon(bounds.getNorth(), bounds.getCenter().lng)
const boundsSouth = new LatLon(bounds.getSouth(), bounds.getCenter().lng)
const boundsEast = new LatLon(bounds.getCenter().lat, bounds.getEast())
const boundsWest = new LatLon(bounds.getCenter().lat, bounds.getWest())
const lat = boundsNorth.intermediatePointTo(boundsSouth, yPercentage).lat
const lng = boundsEast.intermediatePointTo(boundsWest, xPercentage).lng