Last night, I started having this issue in every single Windows 10 app where the app would register the down arrow key being pressed in scrollable areas and would keep cycling down. Restarting didn't fix it and unplugging the keyboard didn't fix it either. Like it would keep scrolling even after I unplugged the keyboard.
I ran a virus scan and a malware scan and both came up negative. This is pretty disruptive because I can't access my mail or use Skype for Windows 10 (I had to reinstall the desktop app). Luckily this issue doesn't seem to happen with the Settings app, but it occurs with every app I've tried so far (Store, Skype for Windows 10, Alarms and Clock, Mail). Does anyone know what's going on and how to fix it?