When using the X11 gtk3 version of gpredict on macOS (via MacPorts in my case) whenever you move the mouse over the gpredict window it turns black.
Unfortunately this is a known problem with XQuartz (X11 server) that doesn't look like it's going to get fixed.
To work around it, use the Quartz version of gtk3 instead.
sudo port install glib2 +quartz -x11
sudo port install libepoxy +quartz -x11
sudo port install gtk3 +quartz -x11
Carl.