I recently switched to Windows 10, and have found that one really annoying habit it has is that it will randomly switch focus without warning. I can be typing an email, scrolling down through the errors in the event viewer to see what vital parts of Windows crashed today and so on, and suddenly I find that my words are not entered into the email, the events aren’t scrolling, etc.
A quick bit of searching revealed that a lot of people have had the same problem, and very few have managed to solve it. The main problem is finding out what gets the focus.
This post explains how I found out.