visualisation - Styling polygon layer as dissolved with QGIS Geometry generator?

The QGIS Geometry Generator seems like a nice way to keep data intact while allowing for some creative visualisations.

I have a layer of tesselating polygons that I would like to style as a continuous surface without internal borders.

To dissolve a number of polygons in Spatialite I would use the UNION function, but Union in the Geometry Generator doesn't give a result, ie:

 union( $geometry, $geometry )

So is it possible to use the Geometry Generator to show a polygon layer as dissolved, and if so, how?

Answer

You can now do this with the geometry generator in QGIS 2.99/3.0 by using buffer(collect($geometry),0) in the expression builder.

The new collect() aggregate function collects all the feature geometries into a multipolygon; buffer() converts it into a single polygon to be styled as such.