Because of rotation and tilt, an extent can never correctly cover the camera's visible area in 3D. We currently offer two possibilities
- SceneView.extent: I assume you are already using that.
- GroundView.extent: similar to the above, but specifically tailored to the ground elevation, making it more precise for some situations.
It might help to know that we are currently working on a property with the visual area. Because this will be provided as a polygon, it will take tilt and rotation into account. This is planned to land this year, potentially in the upcoming 4.30 release.
In the meantime you could increase the size of the extent returned by SceneView using Extent.expand. Computing the extent or visible area yourself is rather complex and I cannot point you to any recourses. Our own implementation takes various factors into account, such as the elevation or visible geometries in the view.