Hello @thomas.f
Since the update to tk version 8.6.15, MacPort has changed the default variant to +quartz. This is causing the compilation problem and the strange Flair appearance.
To fix this, please run the following command to switch to the +x11 variant.
sudo port install tk +x11
After that, please recompile geoviewer and test Flair again.
Please let us know if this resolves the issue.
Cheers, Luigi