Switching apps based on where user looks
My answer falls in the "other suggestions" opening provided.

There's a tool called Isolator that may help you stop mistaking which app is in the foreground (and keep your focus in the right place).  Here's the description from the tool's home page:


  Isolator is a small menu bar application that helps you concentrate.
  When you're working on a document, and don't want to be distracted,
  turn on Isolator. It will cover up your desktop and all the icons on
  it, as well as the windows of all your other applications, so you can
  concentrate on the task in hand.


There's an option to make background windows blurry or semi-transparent, in lieu of simply hiding them.  Witness the blur:


(source)

Reviews elsewhere:


OS X Daily: Focus on Apps Easier &amp; Apply Background Filters with Isolator for Mac OS X
Lifehacker: Download of the Day: Isolator (Mac) &ndash; mentions two similar tools.
MacApper: Anti-Distraction Apps: Keep Yourself On Task &ndash; (ditto)