Using UE 4.14.0 Preview 1, enable forward shading, then change the view mode to quad overdraw. Triangle edges are missing from the display, so only pure overdraw is seen. With forward shading disabled, triangle edges are accounted for as expected.
I see this problem in 4.15.1 also. The cross hair is on a sphere with lots of triangles but it is not showing the edges overdraw. I’m using forward rendering with 4 sample MSAA.
http://i.imgur.com/okaleGd.jpg
This is what it looks like in deferred shading with the same sphere.