[winswitch] Black screen on ElementaryOS

[Mark Messer]

I have an issue with Xpra - as I need a remote screen locally I did
everything in the following order:
xpra start :99
export DISPLAY=:99
someapprun

and in another terminal:
xpra attach :99

And it works almost fine with one small exception - the screen is black. It
reacts to clicks. When clicked top right on black, where the close button
should be the app closes. Tested also an app that has a close button on the
bottom - works the same way. So the screen indeed works. But misses to show
graphics.
I even turned on debug -d all and see that messages re mouse travel well.
Cannot see errors, but maybe they scrolled in a mass of messages.
What else should I check, what switch enable, is it a known issue?