Alt+Tab is window selector Ubuntu and Windows but in KDE it shows the desktop. In Ubuntu Ctrl+Super+D will minimize all Windows and reveal the desktop (with icons).
The OP's solution is to turn off desktop icons using:
gsettings set org.gnome.desktop.background show-desktop-icons false
Now there are no icons on the desktop at all, whether you minimize all Windows or not.
Another option is to install KDE add-on; kwin-addons in [K]ubuntu 16.04:
So in KDE you can change the design of the Alt+Tab application
switcher. On my previous installation, I always used one called "Large
Icons", which was a Mac OS inspired application switcher that only
showed icons. In my installation this switcher is no longer available
(Settings -> Window Management -> Task Switcher). I can only select
Cover switch, Flip switch, Breeze and Breeze dark. Does anyone know
where that nice application switcher went?
The solution (which might be your original problem) could be "Large Icons" as this screen (from the Arch Linux link) shows:
Solved.
Alt+Tab is window selector Ubuntu and Windows but in KDE it shows the desktop. In Ubuntu Ctrl+Super+D will minimize all Windows and reveal the desktop (with icons).
The OP's solution is to turn off desktop icons using:
Now there are no icons on the desktop at all, whether you minimize all Windows or not.
Another option is to install KDE add-on;
kwin-addons
in [K]ubuntu 16.04:From Arch Linux this user writes:
The solution (which might be your original problem) could be "Large Icons" as this screen (from the Arch Linux link) shows: