I've tried to write my own (simple, but with few important features) window manager for X Window System. But after writing base (very minimal) I've got problem with testing. I have tried to test on virtual machine, but this way is uncomfortable. So, how to test window manager without login-out from current session?
You can test your window manager in a nested X server. Xnest has been obsoleted by Xephyr so I would suggest the latter for your experiments.