Version 8.6.1 on Ubuntu 15.02
and version 8.6.6 on Ubuntu 17.04
display box characters.

I tried another window manager on my machine and same issue.

The display problem is looking like a (hard to find) tk bug to me.

Hah!  Compiled with clang (3.5.0) and the display bug has gone away.
Was using gcc 4.9.2.