"vblank wait timed out" triggered by Notes (panel app) install

[JohnAspinall]

Ubuntu Studio 16.04 on Lenovo T520 laptop.

I installed (Panel App -> Panel -> Install New Items) the Notes panel app.
After doing so (and continuing after I removed it) I'm now getting this sort of error in dmesg:

[64397.223784] WARNING: CPU: 2 PID: 3000 at /build/linux-5vkMGy/linux-4.4.0/drivers/gpu/drm/drm_irq.c:1326 drm_wait_one_vblank+0x1b0/0x1c0 [drm]()
[64397.223785] vblank wait timed out on crtc 0

The user-visible symptom is that many apps (Firefox, Thunderbird, LibreOffice) now hang upon launch.
If I poke around in the process tree, all apps are waiting on a subprocess. Killing the subprocess will bring up the app.

Running strace on the processes, the hanging subprocesses have this in common:
 * opening /etc/drirc for read
 * checking for the existence of ~/.drirc (it doesn't exist)
 * a syscall writing to some handle that ends up hanging, e.g.

    ioctl(5, DRM_IOCTL_I915_GEM_CREATE, 0x7ffd90290680) = 0
    ioctl(5, _IOC(_IOC_READ|_IOC_WRITE, 0x64, 0x5e, 0x28), 0x7ffd90290630) = 0
    ioctl(5, DRM_IOCTL_I915_GEM_SET_DOMAIN, 0x7ffd90290630) = 0
    ioctl(5, DRM_IOCTL_I915_GEM_SET_DOMAIN <unfinished ...>

I've attached a larger fragment from dmesg.

[JohnAspinall (jga)]

Also attached strace output from hanging subprocess in the case of firefox.

[Erich Eickmeyer (eeickmeyer)]

Seems like a bug in Xfce, but I'm not sure if it's fixed or not. With 16.04 reaching EOL at the end of this month, I'm moving this to be tracked by Xfce.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Launchpad Janitor (janitor)]

[Expired for xfce4 (Ubuntu) because there has been no activity for 60 days.]