XFCE on cubox comes with a setting such that the tab character switches windows, rather than auto completing.
info on this page:
Without changing any settings, use control-i to autocomplete.
Try using Ctrl-I instead of tab
For changing the setting in the XFCE xml config:
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 behavior and makes it into a switch window key.
On Ubuntu 14.04, the xfce4 somehow resets the xfce4-keyboard-shortcuts.xml file each time you log in, so one needs a method to change the default
the Window manager keyboard shortcuts don't include a setting for the tab-complete Super tab, so it must be edited somewhere else.
Explanation of how to find and change what settings are in the Window Manager->Keyboard tab is here
change "Switch window for same application" to clear from "alt tab"