Update: it worked in a clean profile. Once again, no repro (white circle successfully tracks the cursor, on Windows 10, using today's (2019-08-26) nightly).
Bug 1576524 Comment 7 Edit History
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Update: it worked in a clean profile. Once again, no repro (white circle successfully tracks the cursor in the popup, on Windows 10, using today's (2019-08-26) nightly).