After sr#902488, plasma & wayland are defunct in these aspects:
* thunderbird flickers like mad -- when moving mouse over the window, it is
switching between old and new window buffer
* selection buffer doesn't work between firefox and xterm (select and paste
with middle-button).
* clementine is not shown in the tray.

This fixes all those:
* open a terminal
* export GDK_BACKEND=x11
* clementine &
* thunderbird &
* firefox &

After this all the apps behave as they should. I am not sure if to report this
against the separate components (and leaving this as a meta bug)?