I often use unicode letters in my Coq identifers, to better match what we do on paper. This works fine in Coq, but the syntax highlighting of this extension draws a box around some these characters:

This makes them look like some sort of error, and also makes them hard to read.

In a plain text file these letters look just right, so it seems to be something specific to what VsCoq does in .v files. This seems to be font-independent; I tried two different monospace fonts and they both show this effect.