Unexpected fullscreen on reconnect with SDL applications

[uli42]

Hanno Foest reported via via x2go-user mailing list:

I was running sz81, a ZX81 emulator using SDL, remotely with an IRC
client and noticed, that while it worked nicely if started in a session
using an x2go Linux client, after disconnecting the session and
reconnecting using a Windows client, the application window came up
garbled with display artifacts.

What's more, if I disconnect and reconnect the session using a Linux
client, it somehow manages to blank my complete local display as soon as
the SDL application window gets focus. This confused the hell out of me,
and I first suspected a crash, until I figured out what was going on.
It's really an SDL issue, I verified by running parallax-4.tar.gz from
http://olofson.net/examples.html which displayed the same behaviour.

I would guess x2go has some kind of SDL passthrough mode which is
somewhat buggy and doesn't work with windows. Is that correct, or what
is the expected behaviour?

I managed to replicate the issue locally on a plain nxagent 3.6.99.26 without using x2go at all like this:

1. run nxagent -ac :55
2. in a second shell run DISPLAY=:55 parallax2 -> works fine on the
   nxagent window
3. in a third shell run kill -HUP $(pgrep nxagent) -> window will
   close (resembling session suspend)
4. wait a while, then repeat the previous command -> window will show
   again (resembling session resume) -> no fullscreen, no glitches

Repeating the test with nxagent -ac -55 -R in step 1 (thus simulating
a rootless session) will shows the problem here, too. So it really looks
like a bug in nxagent.