Issue at Hand
You report that you are unable to bind anything to Shift+Tab.
Your issue does appear to be a bug. Here is a link to a potentially related bug report. According to this source, there is an issue with key bindings being set and being forgotten on reboot. There is a potential work around however. I will include links to a stack exchange post that references this bug, as well as a Linux Mint forum post that is related as well.
Newer Possible Solution
Now this is mentioned as a workaround but please try this:
Aleksander V. Dyomin (aleksanderd-e) wrote on 2014-04-19:
I found a workaround: clear before reassign!
To make a shortcut to work ok, select an action you want to reassign and click 'Clear' button until it will really empty. Sometime if there is some (hidden?)duplicates, you will still see some shortcut(the same or different), even you clicked 'Clear' button. In this case just click the button again. Since the action shortcut is really empty, just assign your custom one as usual, and it will work ok :)
In short, open the Windows Manager Keyboard settings menu, then clear out the keyboard shortcuts for the action you wish to map. Then hit clear again, and again to be absolutely sure that all entries are cleared. It appears that there can be multiple conflicting keybindings and that if there is more than one keybinding it does not show up as visible in the menu. Make sure to do it several times, then relaunch the menu and set the desired keybindings. You will have to do this for each keybinding.
If you feel this bug does not apply to you, try these steps:
Look for a
Look for a
<property name="<Shift>Tab" type="empty"> line as well as all desired keyboard properties like
<property name="<Alt>Tab" type="string" value="cycle_windows_key"/> for example.
If any of these lines are missing or they contain the wrong values or are out of order then the desired keybinding will not work. Fixing a missing value assignment should correct the issue.
Old Potential Solution
I will be referencing this stack exchange post regarding changing global keyboard shortcuts, as well as this xfce forum post about changing shortcuts. Please read over them in their entirety before trying anything.
If user GRC's answer is not acceptable then you should try the following steps as recommended by user ToZ on the XFCE forums:
- Make sure that xfsettingsd is running and that you don't have another settings daemon (e.g. gnome-settings-daemon or krunner) running.
- Check that you haven't created any bindings with xbindkeys (look at ~/.xbindkeysrc).
Either create (or remove) the desired shortcut from
~/.xbindkeysrc and run:
killall -HUP xbindkeys
to reload the configuration. You can use the following command to search for Shift+Tab keybindings.
xfconf-query -c xfce4-keyboard-shortcuts -l -v | grep "<Shift>Tab"
This issue appears to be a bug as outlined in this bug report. I have listed a possible work around. Again the links to relevant forum post are here and here. Please read through them in their entirety and verify if they apply.
If these newer steps do not work. Please consider filing a bug report with the respective developers. Here is a link for Linux Mint, and this is a link for XFCE. Document as much as you can of the behavior. If you find a newer solution in this process post an answer here, let me know and I will upvote you.
I would check for any rogue settings managers and either make changes there or purge them from your system. Verify that all the global menus and configuration menus have the correct settings you desire.
However if you are having issues changing application specific keybindings you will need to update your post to include what application you are using. This solution works only for global xfce settings. Please verify if you can set any kind of shortcuts first.
Please comment if you have any questions or issues with this answer. I appreciate feedback to correct any misconceptions and to improve my posts. I can update my answer if you let us know if this is an XFCE or application specific issue.
Best of Luck!