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.
This fix works for me. Thanks tons!!!
参考：https://ubuntuforums.org/archive/index.php/t-1771058.html