Windows Outlook doing an odd thing on 2 monitors

[Shirty]

On my work PC I have 2 monitors, and since this morning for some reason when I have a new email in one window and I start to type it extends the new message window across both screens, which is irritating.

It's almost like a modifier key is stuck down, but nothing is touching the keyboard. Any ideas what might be causing this?

The E key seems to be the culprit, but sticky keys isn't on and the keys all work on Passmark Keyboard Checker.