Now you see it, now you don't

I've made a HUD that includes a bit where one prim only just pokes through another. At my building scale (10x the size I use when wearing it) this poke-through is just one millimeter, so it's reduced to 0.1mm at my wearing scale.

Mostly this works just fine, but I've just discovered that there are some "y" positions for the HUD (moving it around using the green arrow) on my screen where the poking-through prim disappears from view. This effect doesn't happen when changing the "z" position (blue arrow); if the prim is visible at a particular "y" it remains visible whatever the "z", and it also remains invisible at a particular "y" whatever the "z".

I guess this could be down to rounding errors, but I'm puzzled as to why only the "y" position seems to affect this.

Can anyone enlighten me?

when two prim faces occupy the same spot, the viewer is essentially trying to render both faces at the same time. The result is when the view is moved around, sometimes one face shows and sometimes the other face shows. Moving the camera around on an in world rezzed object will produce a jittery flicker effect. Such a small difference 0.01m is enough such that you might get this same effect as well. Try offsetting it more than 0.01m to fix the issue.