I have installed and added all updates for the 2018-04-18-raspbian-stretch image. After installing x11vnc and tightvncserver I have had the following experience with a remote session using Remote Desktop Viewer on a Ubuntu 17.10 desktop.
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Open a terminal window on Maria-Agnesi
ssh from rquint@Maria-Agnesi to pi@Mrs-Lovett
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Started tightvncserver in the terminal on Maria-Agnesi
pi@Mrs-Lovett:~ $ vncserver -geometry 1600x900 :1
New 'X' desktop is Mrs-Lovett:1
Starting applications specified in /home/pi/.vnc/xstartup
Log file is /home/pi/.vnc/Mrs-Lovett:1.log
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In the Rmote Desktop Viewer session I selected Main Menu|Shutdown|Shutdown on pi's X desktop (Mrs-Lovett:1)
No response on the Desktop or in terminal on Maria-Agnesi
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Selected Main Menu|Shutdown|Synaptic Package Manager on pi's X desktop (Mrs-Lovett:1)
Response in terminal on Maria-Agnesi
pi@Mrs-Lovett:~ $ ==== AUTHENTICATING FOR com.ubuntu.pkexec.synaptic ===
Authentication is required to run the Synaptic Package Manager
Multiple identities can be used for authentication:
1. ,,, (pi)
2. root
Choose identity to authenticate as (1-2):
Selected 1 and got response in terminal on Maria-Agnesi
-bash: 1: command not found
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Opened a terminal window on pi's X desktop and ran
pi@Mrs-Lovett:~ $ synaptic-pkexec
==== AUTHENTICATING FOR com.ubuntu.pkexec.synaptic ===
Authentication is required to run the Synaptic Package Manager
Multiple identities can be used for authentication:
1. ,,, (pi)
2. root
Choose identity to authenticate as (1-2): 1
Password: <entered password>
polkit-agent-helper-1: error response to PolicyKit daemon:
GDBus.Error:org.freedesktop.PolicyKit1.Error.Failed: No session for cookie
==== AUTHENTICATION FAILED ===
Error executing command as another user: Not authorized
This incident has been reported.
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If I use an x11vnc session or if I just use a monitor directly with the Pi I have no problem getting the authentication dialogs. I can successfully run sudo apt and sudo shutdown in the terminals. This problem didn't occur in a similar setup in Jessie.