With qml TableView, negative row/columnSpacing values appear to be treated as zero.

This prevents using cell borders without double-width lines appearing between adjacent cells (or, looking at it the other way, half-width lines around the periphery).  

This is unlike with ListView where negative "spacing" is supported;
   spacing: -cellborderwidth
nicely overlaps cells so that borders appear to have uniform width in the interior and edges.

Hopefully TableView can be taught to handle negative spacings in the same way.