It behaves this way intentionally, because once user hovers the scrollbar itself (the one shown by the browser), the mouse cursor leaves the page and no longer can be tracked by the extension. That's why the script catches when cursor leaves the window on the right side, and after some timeout concludes that user hovers the scrollbar and opens the preview.
Unfortunately I couldn't think of a better solution for this, although I agree that it causes false positives which are annoying.
