I do not have enough reputations to post image :(
I google it many times, but the answers are always about the hdmi, mulitple monitor, they are not my problem -- about the Unity
the problem is ,
when I unplug the external monitor (HDMI), the launcher and top menu bar are hidden /disappeared, I can see the desktop background (like the second picture).
and when the mouser click the hidden area, the bars appears(like the first picture).
and when I logout, and login the unity session, the bars are back to work (like the first picture).
the following is the OS info
cat /etc/issue Ubuntu 16.04.2 LTS \n \l uname -a Linux trydofor-5510 4.4.0-66-generic #87-Ubuntu SMP Fri Mar 3 15:29:05 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux