I am working on Ubuntu with virtualbox (on my mac). I have just installed gnome, and it broke a lot of features, among wich the very nice feature of seamless mode. How can I retrieve it ?
Both "Seamless Mode" and "Auto-Resize Guest Display" are advanced features from Virtual Box that are provided from the Guest Additions. These may have been deactivated and need to be reinstalled.
See this questions on details how to install Virtual Box guest additions in Ubuntu:
Usually the guest additions CD will be mounted from the menu entry Devices -> Install Guest Additions in Virtual Box Manager. For a manual installation the guest additions CD can also be found in Mac OS as follows:
you can find this file in the application bundle of VirtualBox. (Right click on the VirtualBox icon in Finder and choose Show Package Contents. There it is located in the Contents/MacOS folder.)Virtual Box User Manual
For recommended procedures to install Virtual Box in MacOS also read more in the Virtual Box User Manual.