Some progress happened in private and I should have been updating this bug report a while ago.
It seems we dropped #883 from the conversation list a while ago accidentally.
Alex has committed a few changes in http://code.x2go.org/gitweb?p=x2goclient.git;a=commit;h=58c2db08167d490797f3... that were subsequently fixed up by http://code.x2go.org/gitweb?p=x2goclient.git;a=commit;h=d220069e8f721b2d4621... and http://code.x2go.org/gitweb?p=x2goclient.git;a=commit;h=465b0309db391a0ae4dd...
These basically start VcXsrv with -nodecoration instead of -multiwindow when a "fake" fullscreen session on one display is requested (i.e., when the window is not rootless and "use whole display" was set in session settings.)
Mike#2 has tested the changes and came up with these results: http://imgur.com/tmYhffj,L7778NZ,kdege2q,slDcGSa#0
Screenshots 3 and 4 only relate to Cygwin's XWin server.
Testing looked good, but another issue resurfaced:
I did discover another issue/limitation though. It happens when we start a "Use Whole Display" session and the X.org server settings are set to "use custom X-Server" but with "start X-Server on X2Go Client start" disabled. The X server specified in that GUI is launched, but with the options determined by X2Go Client rather than the options specified by the user in the GUI. In fact, we need to add a widget and setting for "Use whole display": http://imgur.com/KpYE64y
Thus, there's one more change required within the client's global settings regarding the usage of an external X server.
At the same time, I'd suggest removing the "start X-Server on X2Go Client start" option completely, because the different session types require differently started X server instances.
Mihai