Hi Mike - that's a display artifact / display imperfection which is just a normal side effect of how the display works.

The viewport display has an emphasis on generating an image really quickly, so there are all sorts of little display glitches similar to this which can happen - it's not really feasible for the display to work perfectly because generating the equivalent of a full offline render for every display refresh would make for a very sluggish display.

One of the types of display artifacts is having "bleed through" of edges and curves when they are on thin walls or just fairly close to the surface that should be obscuring them.

The display engine pulls edges and curves forward when they are displayed, so that the edge curves or curves that are drawn right on a surface are not partially obscured by that same surface that they are sitting on.

That forward bias pull can have a side effect of making some curves display as non-hidden when they are just a little distance below a surface. But without that biasing there would be a much worse display artifact of edge curves and curves drawn on surfaces being kind of "stippled" and having parts of themselves displayed as hidden when they are coincident with a surface.

In the future I want to make some tune ups to this area of the display to help reduce some of these display artifacts in some cases, but in general display artifacts like this are just a normal side effect of various shortcuts that are taken to meet the primary goal of having the display work really quickly, so they are really just things to ignore.

- Michael