Hello, KDE lovers!
I am using KDE Neon for about half year now on daily basis for work and a few days ago, my Alt+Tab shortcut stopped working completely. I restarted, tried change it to something and else and then back, but Alt+Tab combination is dead - but any other shortcut is switching windows fine (using Meta+Alt+Z and it is working for now).
Did any of you have similar problem in the past and know how to fix? Reinstall is not a solution for me for now, because it is just too much time and stress of setting everything up again.

Thank you very much!

Are you sure the Alt and the Tab keys are still working? What about Alt+Shift+Tab to walk through windows backward, does that also not work? What about using Tab with the right-side Alt key. Try assigning it to Ctrl+Shift+Tab to see if somehow Alt plus Tab is the problem. Did you install any other programs recently that may be intercepting Alt+Tab?

Alt+Shift+Tab does not work too, but Alt and Tab keys separate are working, I use tab while programming all the time and now I switch windows width Meta+Alt+Z, so both keys are definitely working. When I tried to set Ctrl+Tab shortcut, it worked too, it is just related to Alt+Tab combination.
Also I did not install many programs recently, so I think it is not the case. I remeber this happened to me 2-3 times before and restart fixed it, but now, it is permanent.

Curious problem. Try the following command in a terminal:

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sudo showkey

What if anything gets output when entering Alt, Tab, Alt+Tab, and Alt+Shift+Tab. For some of these I see (also the right Alt and Shift produce different keycodes):

<TAB>
keycode 15 press
keycode 15 release

<ALT>
keycode 56 press
keycode 56 release

<ALT><TAB>
keycode 56 press
^[ keycode 15 press
keycode 15 release
keycode 56 release

Not sure what the ^[ is (probably whatever the terminal interprets the Alt+Tab as), but you can see the ALT pushed first (and held) "56 press", TAB pushed and released "15 press" and "15 release", then ALT release "56 release".

Tried the showkey command, here is the output. Seems to be fine...

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Alt key

keycode  56 press
keycode  56 release
keycode  56 press
keycode  56 release
keycode  56 press
keycode  56 release
keycode  56 press
keycode  56 release

Tab key

        keycode  15 press
keycode  15 release
        keycode  15 press
keycode  15 release
        keycode  15 press
keycode  15 release
        keycode  15 press
keycode  15 release

Alt+Tab

keycode  56 press
keycode  15 press
keycode  56 release
keycode  15 release


I wonder if it has to do with the switcher effect not working or something.
Try fiddling with different visualizations and other settings in System Settings -> Window Management -> Task Switcher.

Thanks for suggestion, but unfortunately, switcher effect does not change anythink. I have a feeling that this has nothing to do with GUI itself but rather somethink is blocking this shortcut. I tried to switch to Noveau driver from Nvidia and it worked, so I suspect it is a GPU driver. But Noveau somehow does not support my monitor resolution, only 640x480 maybe because I use DP instead of HDMI.

This answer I will come with here may have a positive effect the ALT+TAB issue, or none at all. But it did have a positive effect on a related issue: the Task Switcher in KDE 5.15.1 suddenly did not work at all, after an update. It was broken, and the fix that worked is what I write here.

I have just updated to KDE Plasma 5.15.1 (KDE Framework 5.55.0) after not having updated for some months. The way I did it was to open Konsole then type this (and answer with normal password when prompted):

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sudo -i
apt update
apt upgrade
exit

When I then rebooted, even after trying an extra apt update with the --fix-missing parameter, the bottom line task panel was messed up and at best half-working. This, however, when typed into Konsole, solved it:

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pkcon update


Note that one must specifically answer 'y' when prompted here (not just press lineshift). Reboot after this and the task manager/task switcher and so on should be operational again, if the issue was the same as with this (rather massive) installation of KDE Neon that I was running on a powerful HP laptop.

I just clicked on "Default" in Task Changer dialog and Alt-Tab started to work again.

jorgwel wrote:I just clicked on "Default" in Task Changer dialog and Alt-Tab started to work again.

Thanks, fixed for me.

ho yeah ! great ! resetting to default in the task switcher is getting me to a "normal situation" ... great thx ! I was thinking about re-installing (nothing was listed in the general shortcuts )