The Opera browser uses mouse gestures that allow the user to navigate through tabs and tab histories with the help of mouse-clicks and -movements. This can be a hindrance however if you are used to control the browser with its own graphical interface and often just play around with your mouse out of boredom.
Disable Opera mouse gestures
To disable these gestures you simply need to open an Opera window and go to Settings > Preferences > Advanced > Shortcuts and uncheck the Enable mouse gestures checkbox.