There is a column of pixels on the right side of Opera one's vertical scrollbar that behaves like the title bar when clicking and dragging. Clicking and dragging the scroll bar block in Opera one sometimes pulls the window out of page and into windowed mode just as happens when clicking and dragging down on an empty spot on the title bar.
Behavior is inconsistent. Most of the time the window scrolls as expected but occasionally the window is just moved instead.
This behavior is easily replicable by throwing the mouse to the right side of the screen and attempting to click and grab the visual scroll indicator block to scroll the window up or down. imgur screen capture