http://ubuntuforums.org/archive/index.php/t-1771058.html
I accidentally discovered a fix for this while trying to solve a
different
problem.
edit
~/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-keyboard-shortcuts.xml
find
the line
<property
name="<Super>Tab" type="string"
value="switch_window_key"/>
and change it
to
<property name="<Super>Tab"
type="empty"/>
reboot or whatever and then tab will
work properly!
I have no idea why but when using vnc
this file seems to override tab‘s normal behaviour and makes it into a
switch window key.
解决VNC中tab键无效的方法,布布扣,bubuko.com
原文：http://www.cnblogs.com/JimMoriarty/p/3758524.html