Oh yes thats a great answer.

No the Problem is a bit different.

Lets make an example.

Imagine, open windows: Opera and File Explorer from the OS.
Then you are in Opera. Press Alt+Tab to change to Opera. Opera keeps staying in front, and the Window look changes to inactive. In the Taskbar the file explorer gets active but does not come to front.

Same problem can be seen with overlapping windows.
I can reproduce the Problem on 3 Windows 8.1 machines.
The Problem keeps with other applications after closing Opera, a reboot fixes this. Problem is back again after Opera beta was started.

First of all I think Opera is simply the best Browser out there. But for a real Beta it is too complicated to give Feedback. I couldn't find a simple Button "Bug Report" in the Menu.

So my Problem I expereiece science the new Version:
Version: 28.0.1750.36 - Opera ist auf dem neuesten Stand
Update-Stream: beta
Betriebssystem: Windows 8.1 64-bit (WoW64)

is that when I have multiple Windows opened, it is difficult to switch between them.
Opera does not come to front if I change to the window.
Opera does not go to back if I activate an other window.
This happens nearly every time. Multiple times of Clicking on the Application in the Task bar helps.
This Problem occurs with 2 Windows of Opera open, and with other Applications.

Best Regards