Have a look at the method implementation of

GeoDataLatLonAltBox AbstractProjection::latLonAltBox( const QRect& screenRect,
const ViewportParams *viewport ) const

This methods "samples" coordinates along the border of the viewport. It does it first for the

upper and lower border and then for the left and right border.

We would like to have a method

GeoDataLinearRing AbstractProjection::boundingLinearRing( const QRect& screenRect,
const ViewportParams *viewport ) const

which samples the border with the given latLonAltBoxSamplingRate clockwise resulting in a LinearRing ("Polygon").

This boundingLinearRing should be visualized like this (in the mockup the boundingLinearRing

looks like a circle while in reality it's some irregular shape most of the time).

For the first version of the patch this feature should be enabled by default. Adding settings and tweaks and porting the AbstractProjection::latLonAltBox method to use the boundingLinearRing() method will be part of follow-up-tasks.