Xorg 1.20 is planned for RHEL 7.6 and tigervnc needs to be rebuild, including some minor tweaks to build.
Fixed with tigervnc-1.8.0-6.el7.
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.