Patch to tigervnc.Slackbuild in -current -- Include new patch from mainline Slackware tree
 

--- tigervnc.SlackBuild.dist 2020-06-03 16:31:21.838623216 -0500
+++ tigervnc.SlackBuild 2020-06-03 16:31:32.582856380 -0500
@@ -105,6 +105,10 @@
# Explicitly link against libpng to prevent linking errors:
zcat $CWD/patches/tigervnc13_link_png.patch.gz | patch -p1 --verbose || exit 1

+# https://github.com/NixOS/nixpkgs/issues/78627
+zcat $CWD/patches/tigervnc.xorg-server-1.20.7.diff.gz | patch -p1 --verbose || exit 1
+
+
[ "$DO_APPLET" = "YES" ] && CMAKE_JAVA="ON" || CMAKE_JAVA="OFF"
# Explicitly put the java applet into a directory named 'tigervnc':
sed -i -e 's#/vnc/class#/tigervnc/class#'g $(grep -rl vnc/class .)

I remember getting into a muddle with that one in the most recent update. I must have missed that patch!
Thanks, I'll add it to the next batch.