Since a recent update (within the last 24-48 hours), my arrow keys are malfunctioning. The Down arrow key disables the touchpad (complete with OSD) - in Konsole, trying to go down the list in krunner, trying to go down browser address bar history... it's a mess. I had to carefully go into System Settings and set a hotkey to turn the trackpad back on - the laptop's Trackpad toggle key (Fn-F7) does not work. Note that this only affects the 4-way arrow keys. The arrow keys on the numpad are unaffected.
The DEL key is now a shortcut to turn the trackpad back on. I didn't set it.
Corrected in upgrade batch distributed approximately 1300 CDT - 5.50git.20180913xxx...