Async notificatiions appear on-screen while it is locked

The problem lies in the interaction of xscreensaver and skype.

I have long-running text chats. When the screen is locked and blanked, newly arriving skype chat notifications still appear on the screen. This is a privacy leak as all new messages are visible.

Skype version 4.3.0.37.

 lsb_release -rd
Description: Ubuntu 14.04.4 LTS
Release: 14.04

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: xscreensaver 5.15-3ubuntu1
ProcVersionSignature: Ubuntu 3.16.0-62.83~14.04.1-generic 3.16.7-ckt23
Uname: Linux 3.16.0-62-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.19
Architecture: amd64
CurrentDesktop: XFCE
Date: Wed Mar 9 07:54:59 2016
InstallationDate: Installed on 2015-06-03 (279 days ago)
InstallationMedia: Xubuntu 14.04.2 LTS "Trusty Tahr" - Release amd64 (20150218.1)
SourcePackage: xscreensaver
UpgradeStatus: No upgrade log present (probably fresh install)