For archival purposes / convenience, here's a copy of https://lcamtuf.coredump.cx/whack/ (lightly edited to use an inline data URI instead of an external PNG, to make it standalone). I can still reproduce the issue with this testcase and latest Nightly. The mole shows up in a different location depending on whether I've e.g. visited https://twitter.com/lcamtuf or https://www.cnn.com/ or presumably the rest of the URLs in the testcase (and clicking the mole reveals its position and hence my browsing history to the page).
Bug 1293420 Comment 16 Edit History
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For archival purposes / convenience, here's a copy of https://lcamtuf.coredump.cx/whack/ (lightly edited to use an inline data URI instead of an external PNG, to make it standalone). I can still reproduce the issue with this testcase and latest Nightly. The mole shows up in a different location depending on whether I've e.g. visited https://twitter.com/lcamtuf or https://www.cnn.com/ or presumably the rest of the URLs in the testcase (and hence, clicking the mole reveals its position and hence this information about my browsing history to the page).