I also have this issue and it drives me insane! I have this on Windows 7 64 bit, 16 GB RAM and i7 3.6 GHz.
I usually have loads of tabs opened, sometimes I have 2 or 3 Opera windows opened. Barely 1/3 of my memory is used, so there is no reason for Opera to act like that.
Just to make it clear, the input in the DOM is not working (not sure if CTRL+V works), but the address bar is fine, and mouse gestures do not respond. I have just a few widgets installed, browser plug-ins still work.
FIX THIS ISSUE, OPERA and bring back good old tab stacking!