We used to use Horizon client 3.5.2, it worked fine. After we updated to Horizon client 4.xx(have tried 4.0.1, 4.1.0, 4.2.0), all these versions have the same issue as follows:
If you try to connect a Horizon server with a computer with Horizon 4.x on, which has dual monitors and using VIA chip, after you used any of applications from the server, the connection window will become black, you can still move it, but you can see nothing on it.
However it works fine with single Monitor. Also tried with Computer with Intel chip on, there is no such Problems.
Don't know if this is a bug or an known issue, but we are looking for a solution for it. It will be very appreciated, if someone here can give some advices.