If they have a million users, those users log millions of ‘test’ hours each day on a million different machines.
Microsoft’s testers will have fewer repeated tests in their tests, but their users, combined, likely hit untested paths every day.
Reading the thread, not all users have the problem, so this issue seems setting related.
It also may require testing by actual humans (if they find the cause, the engineers may write a test with a heuristic such as ‘the hit area must be at least 3 × 3 pixels for all of these permutations of settings’, but that would be a heuristic, and won’t guarantee that test to catch all future similar problems)
Fascinating comment. Person doesn't know how to correctly use a mouse and it's DPI settings.