odd screen corruption in VNC

[Glenn Holmer <cenbe@kolabnow.com>]

Setting up VNC, both machines running Debian Stretch with MATE (no VMs
involved). Using tightvncserver, and xtightvncviewer as well as vinagre
as client. In both cases I see some corruption of the upper MATE panel
with red circles containing a number or letter. Anyone have any idea
what this is from? The remote desktop appears to function normally in
every way I've tested.

Here's a screenshot:

https://www.lyonlabs.org/panel-corrupt.png

-- 
Glenn Holmer (Linux registered user #16682)
"After the vintage season came the aftermath -- and Cenbe."