Hi Alex,
I spoke with Jon Turney (jturney), the Cygwin XWin maintainer on #cygwin on Freenode. I also tested some options.
It sounds like the best approach to making a "full screen" secondary monitor work is to launch VcXsrv (or Cygwin XWin) with these arguments: -nodecoration -screen 0 @2
For tertiary monitor, use @3 instead.
This launches Cygwin XWin in root window mode (the default) rather than -multiwindow mode. -nodecoration hides the window decoration from the Windows OS.
Can you please test this out and see if it meets your needs? If so, it should be a simple change to x2goclient to launch VcXsrv this way.
Also, FYI:
It is true that if you launch Cygwin XWin or VcXsrv in fullscreen mode, then it will always use the primary display, even when you tell it to use the secondary: -fullscreen -screen 0 @2
However, by telling either X server to use its older rendering engine, you can solve this problem: -fullscreen -engine 1 -screen 0 @2
I do not recommend this approach for x2goclient though because if you click out of the X server, then the X server minimizes.
Note: these are the engines: -engine engine_type_id Override the server's automatically selected engine type: 1 - Shadow GDI 2 - Shadow DirectDraw - obsolete 4 - Shadow DirectDraw4 Non-Locking
Engine 4 is the default. Engine 2 is not available on VcXsrv. Jon acknowledges that there is a secondary monitor limitation/bug with Engine 4.
-Mike#2