Maybe it is related with this bug. When I try to see taskbar thumbnails,
they appear empty (screenshot1).
** Attachment added: Screenshot1
https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bug/601530/+attachment/1719195/+files/screenshot1.png
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And when I show all apps using Showing Windows, they dissapear
suddenly, showing their name only (screenshot 2)
** Attachment added: Screenshot2
https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bug/601530/+attachment/1719196/+files/screenshot2.png
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[965gm]
