upgraded to Ubuntu Mate 19.04. Now any open window will make panels invisible.

[Bodo Brance]

Did a do-release-upgrade from 18.04->18.10->19.04. Upgrade succesfull.
Login works fine. Panels are shown. Background image is OK. Shortcuts and documents on desktop are shown OK.
But the following makes all panels invisible:
- opening any window/graphical program from the panel->Applications->whatever.
- clicking on any part of the panel, except
          - the calendar applet
          - the "switch keyboard layout" button
          - clicking on the "switch workspace" button
- hovering the cursor over any part of any panel.

You can still see the space where they should be, but only the background image appears that would normally be under the panels.

Clicking blindly in the panel areas works, the results are what would happen if that part of panel were visible (like the System menu).

This does not allow me to see at a glance what graphical programs are opened.
Alt-Tab between multiple open windows seems to work fine.

[Bodo Brance (bodobr)]

So this happened after the final upgrade and a reboot.
Did a second reboot a few hours later and the issue is gone!.

Now I wonder, what could have happened that it appeared in the first place?

[Launchpad Janitor (janitor)]

[Expired for Ubuntu MATE because there has been no activity for 60 days.]