Check every button, the scroll wheel, and whether clicks are doubling.
Click the left button slowly about twenty times. A count that keeps climbing points to a worn switch.
Click inside the pad below to test each button, spin the wheel to test scrolling, and watch for the one fault that is easy to miss: a worn switch that sends two clicks when you only pressed once. The tester times the gap between clicks and flags anything suspiciously fast to have been deliberate.
This tests buttons and scrolling only. It deliberately does not report a polling rate: browsers group mouse movements together before handing them to a web page, so any polling-rate figure measured this way is a guess presented as a measurement. A number that might be wrong is worse than no number.
Those buttons are bound to browser navigation by default. The test pad blocks that while your pointer is inside it, so keep the pointer within the pad. If they still navigate, your mouse software has them mapped to keyboard shortcuts rather than real mouse buttons.
It counts times the same button reported twice within a window too short to be human. One or two over a long session can be you clicking quickly. A count that keeps climbing while you click slowly and deliberately points to a worn switch.
Because a browser cannot measure it honestly. Movement events get batched together before a page sees them, so what you would actually be measuring is the browser's delivery rate, not the mouse's report rate. Tools that show a number here are showing you an estimate.
Inside the test pad the menu is suppressed so the click can be counted. Outside it, the normal menu behaviour returns.
That is normal and varies by operating system and mouse. Some report smooth scrolling in small increments, others one large step per notch. What matters is that both directions respond.
Post it in the forums and someone who has had the same fault will usually know whether it is worth cleaning, repairing or replacing. You do not need an account to read, and it is free to join.