Perhaps the browser window is no longer the focus for mouse actions. By clicking somewhere in the browser window (e.g. scrollbar, tab, titlebar) the focus is restored and the mouse resumes control for that window.
Maybe test the theory by clicking on a blank section of the page instead of the scrollbar to see if the same outcome (return of control) is achieved.
How old is the mouse? Perhaps he needs a different type of cheese. I had problems with my mouse and posted the question on here. Techie guy Rojash from on here, suggested I changed my mouse. He was so right, all the problems disappeared.
Good question oldgit, cus my little fellar jumps around all over the screen sometimes and I have to `wiggle it` about to find it!!!......yeah, that remark will bring some funny comments.lol