ALT+Tab no longer switches applications since upgrade to 18.04

In the Settings > Devices > Keyboard menu, you will find a list of keyboard shortcuts. Under the Navigation section, you can find the relevant functions and their associated shortcuts, and change them as you see fit. You're looking for the one named Switch applications.

Run gnome-control-center aka the Settings application.

Remove the shortcut for Switch applications by clicking on it and deleting it with the backspace key, now press the Set button.

Now set Switch windows by clicking on it, then press Alt + Tab and click the Set button.

Turns out the configuration just needs to be updated in settings.

Open Settings > Devices > Keyboard and under Keyboard Shortcuts select Switch applications and update it with left Alt+Tab.

Both Alt+Tab combinations should now work.

If my understanding of the problem stated here is correct try the following solution. It is a three stage process

Stage-1:
(1) Open Settings > Devices > Keyboard and under Keyboard Shortcuts
(2) Remove the shortcut for Switch applications by clicking on it and deleting it with the Backspace Key, now press the Set button.
(3) Remove the shortcut for Switch windos directly by clicking on it and deleting it with the Backspace Key, now press the Set button.

Stage-2:
(1) Create a new shortcut for Switch applications by clicking on it and then press Alt + Esc and click the Set button.
(2) Create a new shortcut for Switch windos directly by clicking on it and then press Super + Tab and click the Set button.

What we have achieved so far is simply swapping the key combinations for the following actions
Switch applications and Switch windos directly
Now let us proceed to the next stage

Stage-3:
(1) Create a new shortcut for Switch windos by clicking on it and then press Alt + Tab and click the Set button.

Voila
You have restored the behavior of ALT + TAB Key to what is required in this post. If for any reason things are not working you can execute the steps in Stage-0 (mentioned below) and then redo steps in Stage-1 and Stage-2

Stage-0:
(1) **Open Settings > Devices > Keyboard**
(2) In the top of the screen you can see the Reset All... button and then click it
(3) It will ask for confirmation. Click Reset All
Caution: TL;DR
The steps in this stage will remove any key customization you might have done in the past before you following the steps in this thread

That's all folks
Have a great time playing with keyboard shortcuts

It is also possible to use Alt + Esc

Somehow maintainers think that this is better. However I always enable Alt + Tab

To get the same settings in 18.04 where alt+tab switched apps and alt+` switched windows within the same app:

Under Settings - Keyboard Shortcuts

In the Navigation section

• Click Switch applications and hit alt+tab
• Click Switch windows of an app directly and hit alt+`

Xubuntu 18.04 Lost Alt-F4 Log Out/Restart/Shut Down? This applies to Xubuntu (xfce) only. It may apply to more than 18.04. In a terminal execute xfce4-session-settings command to open the 'Session and Startup' panel. In the general tab, Logout Settings make sure that 'Prompt on logout' is TICKED. If this is not ticked then the command xfce4-session-logout will execute immediately. If it is ticked then a pop-up window will appear offering options Log Out Restart Shut Down or Cancel. If Alt-F4 does nothing, then create a keyboard short-cut for it: Open the mouse menu, select Setting, Keyboard and then the Applications Short Cuts tab. Click +Add at the bottom right. In the Shortcut Command tab, in the Command box enter xfce4-session-logout and click OK. Then hold down the Alt key and tap the F4 key. Close the panels. Now type Alt-F4 and the Log Out/Restart/Shut Down option panel should appear.

My problem was that in Window Behavior policy has been set to Focus under mouse.

I switched it to Focus follows mouse and Alt+Tab works alright now.

More on it can be read here: https://forum.kde.org/viewtopic.php?f=111&t=88727