@Jacques Lucke (JacquesLucke) The reported GL_VERSION is wrong actually, it's the output given by glxinfo for a non-core context and Mesa (at least for Intel) acts conservatively when asking for a GL context and reports the lowest version possible matching the requested context settings, which here is 3.0. Asking for a core context yields 4.2 (Core Profile) Mesa 18.3.1 but all the required extensions up to 4.2 are still accessible with lower versions.

After some digging it appears the driver chokes on primitive restart indices; the index buffer for the UV map of the default scene cube is
0 1 2 3 0 255 4 5 6 7 4 255 8 9 10 11 8 255 12 13 14 15 12 255 16 17 18 19 16 255 20 21 22 23 20 255 where 255 is set as restart marker.
The weird thing however is if I get the index buffer content (to get the data above) using glGetBufferSubData, restart markers start working correctly and ghost edges disappear. Will investigate further as this is very likely the sign of a driver bug.