With kernel 3.17.0-031700-generic:

- 1st time I plugged in the DisplayPort connector, nothing happened (bug
1263681).  I pressed Fn-Home as a workaround and external monitor got
detected, but internal monitor went black (no picture, no backlight).
Pressing Fn-Home again turned it on, at minimum brightness.  Fn-Home/End
brightness control keys continue to work.

- 1st unplug changed nothing

- 2nd hotplug was detected instantly, internal LCD went black again.
Fn-Home was required again to bring it to life.

- 2nd unplug changed nothing

- 3rd hotplug was same as 2nd.

I can bring the internal LCD into the no-picture-no-backlight state by
pressing Fn-End until I reach minimum brigthness.  This is something
new: with older kernel versions minimum brightness still had a visible
picture.

Conclusion: latest kernel has one half of the bug (DP hotplugging drops
screen brightness to minimum level) but so far hasn't shown the other
half (brightness keys don't stop working).

** Tags added: kernel-bug-exists-upstream

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https://bugs.launchpad.net/bugs/1378863

Title:
  ThinkPad X220: unplugging DisplayPort breaks backlight (intel video)

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