How can I tell QGIS to render a feature outside map canvas?

Question

I have a point layer with one point in QGIS. Instead of rendering its real coordinates, I use it to generate a geometry-generator-based point always in the middle of map canvas (geom_from_wkt('POINT('||x( @map_extent_center )||' '||y( @map_extent_center )||')'))

If I move the original point outside map-canvas the symbol in the middle of the map-canvas is not rendered any longer too.

Is there a possibility to force QGIS to render this geometry-generator-point when its original coordinates are outside map-canvas ?

A virtual layer (as described in Displaying generated lines out of map extent using QGIS?) does not help, because virtual layers do not work with variables like @map_extent_center)

Answer 1

If instead of using a sacrificial point for your layer, you use a polygon, and again use the geometry generator trick to create a central point, then you can make the polygon arbitrarily large (even the whole globe in your project CRS) and it will always render the map-centre point because the polygon will always overlap your view.

Here's a sacrificial polygon with two layers, the red map-centre spot and an outline layer for clarity.

I can zoom in and the red spot is still centre even when the polygon outline isn't visible:

Here's my geometry for that layer:

Note the layer is in EPSG 3857 to match the project CRS.

(Also, using @map_extent_center for your geometry generator is quicker than constructing a WKT string.)