This has been a problem since I had Windows 8 on a very expensive laptop. I’ve heard of it on Windows 7, but never had the pleasure of being introduced to the problem. After I upgraded to Windows 8.1, I still had the problem. The first time this happened was particularly embarrassing. I popped up while teaching a Windows class online. Of course a new bug pops up when you are in front of the public. Here is what is going on.
I open multiple windows, like most over achievers. All of the sudden, one window will not lose focus and fall behind the others. Up until now, the only way to work around this issue was to do one of two things. Minimize the problematic window or close and re-open the application. Since I like to switch between applications using Alt-Tab, this was a no go for me. Unfortunately, it was my only option.
I just discovered this fix. With the problematic window in focus, press Ctrl-Alt-Esc. No idea why, but the Window started to play nice again. Give it a try. I am a lot happier now.