Bug#848134: xfce4-terminal: Weird buffering problem
VTE (libvte-2.91-0) recently received lots of changes to its drawing
component. There were bugfixes around rendering, especially in
non-compositing mode  and many simplifications and cleanups 
released in version 0.56; followed by even more simplifications and
cleanups  for version 0.58.
I'm pretty certain that 0.58, if not 0.56 already, fixes the issue.