The current version of xpra has a bug which prevents the shadowing functionality from working. This was fixed in the following version (0.14.21) but the product is now up to 0.14.25 in the LTS branch.
A commit references this bug: Author: swills Date: Sun Jun 7 12:21:00 UTC 2015 New revision: 388692 URL: https://svnweb.freebsd.org/changeset/ports/388692 Log: x11/xpra: update to 0.14.25 PR: 200626 Changes: head/x11/xpra/Makefile head/x11/xpra/distinfo head/x11/xpra/pkg-plist