Black field with rounded white frame suddenly on my desktop describing everything I am doing. When a page loads it cycles a frame over each button/tab/field and doesn't allow me to scroll down the page (it jumps back to what it's highlighting at the top)

After about an hour without any response to my question, I've discovered that it was the VoiceOver feature that had inadvertently been activated. The sound was off and I didn't realize it was speaking all the functions and text as well!

To anyone else who might stumble upon this with the same problem, I went to "System Preferences" then "Accessibility" (under "System") then "VoiceOver" and clicked off the checkmark next to "Enable VoiceOver". The feature is turned on by Toggling Command F5, which must have inadvertently happened when moving or cleaning the keyboard.

And I imagine the point then is that you can also just toggle Command F5 again to turn it off instead of going through the preferences panes!