I would suggest a severity of S2 for this; even though there is technically a workaround (try the scroll gesture again and it works the second time), the issue is quite noticeable (basically _every_ other upwards scroll gesture fails), affects a core interaction mechanism (scrolling), and affects a common code pattern used on a variety of websites.
Bug 1847305 Comment 2 Edit History
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I would suggest a severity of S2 for this; even though there is technically a workaround (try the scroll gesture again and it works the second time), the issue is quite noticeable (basically _every_ other upwards scroll gesture fails), affects a core interaction mechanism (scrolling), and is triggered by a common code pattern used on a variety of websites.