Good news ! This bug has been fixed by the nvidia non-free driver update 340.65-1: http://metadata.ftp-master.debian.org/changelogs//non-free/n/nvidia-graphics-drivers/nvidia-graphics-drivers_340.65-2_changelog " - Fixed a regression that caused displays to be detected incorrectly on some notebook systems. (Closes: #770798, #765726)" Obviously, the latest intel driver had incompatibilities with the nvidia driver. This bug could be closed.