I think I'm able to reproduce more or less this same bug. For me, it occurs only when I use gnome-control-center to switch from VGA-off-and-LVDS-on directly to VGA-on-and-LVDS-off. If I start with both displays on, and disable LVDS, it works fine. If I use xrandr, it works fine. See bug #737891 (set as a dupe of this one now) for details.
For diagnosing, I used 'xrandr --verbose', which indicates some difference in how the crtcs are being mapped. I also enabled graphics debugging (sudo xdiagnose, first checkbox, apply, reboot), then collected dmesg before and after doing the switch, once with gnome-control-center, and once with xrandr. I will attach both of these files. (The logs fill with pipe A underrun messages which I believe to be irrelevant so have filtered those out for clarity.) ** Attachment added: "Broken case - using gnome-control-center" https://bugs.launchpad.net/ubuntu/+source/gnome-control-center/+bug/828623/+attachment/2467494/+files/dmesg.broke.clean.txt -- You received this bug notification because you are a member of Ubuntu Desktop Bugs, which is subscribed to gnome-control-center in Ubuntu. https://bugs.launchpad.net/bugs/828623 Title: [Regression] gnome control centre does not allow you to disable the laptop screen while leaving an external DisplayPort connected screen working To manage notifications about this bug go to: https://bugs.launchpad.net/gnome-control-center/+bug/828623/+subscriptions -- desktop-bugs mailing list desktop-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/desktop-bugs
