This little menu-bar utility (or NOT in the menu bar if you don’t want it there) essentially replaces all of the following:
* Focus app (that mostly dims other stuff so you stick to what you’re working on)
* App highlighter that more clearly indicates which app is the current input focus, without excessively dimming others you need to read from (most Linux desktop environments have done this for years)
* Theater-mode app that blacks out the rest of the display(s), when you’re watching a movie
I replaced three other apps when I started using this.
I especially like that it can be quickly changed to show only one window or all windows of the frontmost app, and to adjust the extent to which others are dimmed. When I’m working on documents or code development, I keep the dimming very subtle and highlight all windows of the current app, but when watching a TV episode on my Mac, I make it near-black and applicable to every window but the front one.
Because it’s just inserting a layer with configurable opacity between the top window/app and everything else, not selectively "disappearing" all other windows, it works remarkably smoothly and quickly.
This app plays well with the superb app f.lux (http://www.macupdate.com/app/mac/37261/f-lux), and can be used with Honer (https://github.com/puffnfresh/Honer.app), though the latter is a bit buggy, and I only use it when working with a bunch of similar-looking windows, like six Terminal windows at a time.