ROS via VNC "XKEYBOARD extension not present"

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I'm setting up a new Ubuntu server on Digital Ocean to use for a hosted ROS development environment. Here's what I have:

• ubuntu 18.04
• ROS melodic
• xfce4 desktop
• tightvncserver

I create a tunnel from my Mac to that server with SSH:

ssh -L 5901:127.0.0.1:5901 -C -N -l <usermame> <server ip>

I use a VNC Cliend (VNC Viewer) to connect to the Ubuntu server. I see the xfce4 desktop. I know that I have a pretty good thing going because I can run FireFox (a heavy Gui app). Then I try the most minimal bit of ROS:

rosrun rqt_console rqt_console

and I get this error message:

Qt: XKEYBOARD extension not present on the X server (see attached image)

I know this is maybe not a ROS problem. It is probably either a VNC or Ubuntu configuration detail. But I thought that this community might have some experience with it. I've posted this question on StackOverflow as well. Thanks

Qt: Keyboard extension not present on the X Server