In a dual monitor environment where separate Xfce4 desktops are running on each monitor, you can start Synaptic in either desktop environment, but if you start it on display :0.1, it shows the Authenticate pop-up window on display :0.0. Once the password is entered, the main window will then appear on display :0.1. All the components of a given program should be display-environment-aware.
There are a number of system applications that have similar problems. I'll report them as I continue to encounter this problem.
ProblemType: Bug
DistroRelease: Ubuntu 17.04
Package: xorg 1:7.7+16ubuntu3
ProcVersionSignature: Ubuntu 4.10.0-33.37-generic 4.10.17
Uname: Linux 4.10.0-33-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia_drm nvidia_modeset nvidia
ApportVersion: 2.20.4-0ubuntu4.5
Architecture: amd64
CurrentDesktop: XFCE
Date: Tue Sep 12 11:21:53 2017
ProcEnviron:
LANGUAGE=en_US
PATH=(custom, no user)
XDG_RUNTIME_DIR=<set>
LANG=C.UTF-8
SHELL=/bin/ksh
SourcePackage: xorg
Symptom: display
UpgradeStatus: Upgraded to zesty on 2017-05-27 (108 days ago)
Status changed to 'Confirmed' because the bug affects multiple users.