Ok, I have no DeviceFeatureLevel in registry (HKEY_USERS\.DEFAULT\Software\Microsoft\Windows\DWM) I create DeviceFeatureLevel (REG_DWORD 32 bit) and 9300 in hexadecimal (37632 in decimal) : same problem ... EDIT : it's OK after Reboot Thanks !
Ok but how activated this in virtualbox ? I have configured display in virtalbox like that : But it is the same ... Remark with WinaeroGlass (Aero8Tuner + bLend), transparency works very well :