On an old computer running KUbuntu 17.10 (that's KDE + Ubuntu 17.10), somehow, I changed the keyboard shortcut for showing all desktop windows. (The default is Ctrl+F8.)
I just installed the same OS on a new computer, and cannot for the life of me figure out how I changed that shortcut. A lot of Googling hasn't gotten me any closer.
Found it. The magic phrase is "desktop grid", and it is found under System Settings -> Desktop Behavior -> Desktop Effects. (Everything else having to do with virtual desktops is found under System Settings -> Desktop Behavior -> Virtual Desktops.)