When any user is logged in in the guest Debian the following happens, both in windowed, seamless and full screen mode:
Cut and paste from host to host works OK (cut and paste worka in/between any osx application) When guest Debian is running but no user has logged in: I write down again what I 'm experiencing: During the test I have checked the status of Captured and uncaptured, as per your suggestion and verified that was congruent.
Anyway I repeated the test in windowed mode also (sorry for having mistaken with seamless). I was indeed using the left Apple Key to switch.
