I went through the APT history log file (/var/log/apt/history.log, list limited to interesting/related packages only):New packages
virtualbox:amd64 (4.3.26-dfsg-3, 4.3.26-dfsg-3+b2)Removed packages
virtualbox-qt:amd64 (4.3.26-dfsg-3, 4.3.26-dfsg-3+b2)
x11-common:amd64 (7.7+8, 7.7+9)
xorg:amd64 (7.7+8, 7.7+9)
xserver-common:amd64 (1.16.4-1, 1.17.1-2)
xserver-xorg:amd64 (7.7+8, 7.7+9)
xserver-xorg-core:amd64 (1.16.4-1, 1.17.1-2)
xserver-xorg-dev:amd64 (1.16.4-1, 1.17.1-2)
xserver-xorg-input-all:amd64 (7.7+8, 7.7+9)
xserver-xorg-video-all:amd64 (7.7+8, 7.7+9)
xserver-xorg-video-ati:amd64 (7.5.0-1, 7.5.0-1+b1)
xserver-xorg-video-cirrus:amd64 (1.5.2-2+b1, 1.5.3-1)
xserver-xorg-video-fbdev:amd64 (0.4.4-1+b2, 0.4.4-1+b3)
xserver-xorg-video-intel:amd64 (2.21.15-2+b2, 2.99.917-1)
xserver-xorg-video-mach64:amd64 (6.9.4-2, 6.9.5-1)
xserver-xorg-video-mga:amd64 (1.6.3-2+b1, 1.6.4-1+b1)
xserver-xorg-video-neomagic:amd64 (1.2.8-1+b2, 1.2.9-1)
xserver-xorg-video-nouveau:amd64 (1.0.11-1, 1.0.11-1+b1)
xserver-xorg-video-openchrome:amd64 (0.3.3-1+b2, 0.3.3-1+b3)
xserver-xorg-video-r128:amd64 (6.9.2-1+b2, 6.10.0-1)
xserver-xorg-video-radeon:amd64 (7.5.0-1, 7.5.0-1+b1)
xserver-xorg-video-savage:amd64 (2.3.7-2+b2, 2.3.8-1)
xserver-xorg-video-sisusb:amd64 (0.9.6-2+b2, 0.9.6-2+b3)
xserver-xorg-video-tdfx:amd64 (1.4.5-1+b2, 1.4.6-1+b1)
xserver-xorg-video-trident:amd64 (1.3.6-2+b2, 1.3.7-1+b1)
xserver-xorg-video-vesa:amd64 (2.3.3-1+b3, 2.3.3-1+b4)
xserver-xorg-video-vmware:amd64 (13.0.2-3.1, 13.1.0-2)
I have two theories here:
- (less probable) The removed modesetting package caused Xorg not to detect the video hardware in VirtualBox and installing guest additions fixed it.
- (more probable) The virtualbox package update caused incompatibility with the Xorg-supplied drivers for VB-emulated hardware... and installing guest additions fixed it.
I would probably go with the second one here. The solution is... to install the guest additions and make sure that the drivers are 100% compatible with the VM (the Xorg bundled drivers might or might not be compatible with different VB versions; they're not developed by the VB devs, but by Xorg devs).
Maybe we should check if we're in an VirtualBox environment and install guest additions in Sparky's installer. It's not a trivial issue, but I'll add that to my "research" list.
As for the LightDM problems: there were some config file changes (the option to display or not the list of available users has been commented out), so maybe that's the problem here. I discarded the new config file and retained the old one and didn't have any problems with LightDM whatsoever.