Let's assume that I have a Windows 10 VM running under Qemu, there is a way to forward to X11, the windows application. Something like I start the virtual machine, and I can see only the program without seeing the whole windows desktop.
I manage to achieve this using
freerdp with the
/app option like for example
And on the guest you should add this to the registry:
Windows Registry Editor Version 5.00 [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Terminal Server\TSAppAllowList] "fDisabledAllowList"=dword:00000001
It requires the installation of Virtualbox Guest Additions or VMWare Tools on the guest OS but this does exist, just not through the X protocol. It's also not entirely reliable and you may never get it to work, depending often on how much the client OS uses GPU features. On Virtualbox this is called "seamless mode" and is a transparent-window-based alternative to fullscreen. VMware has a similar feature called Unity Mode which I have no experience with.