(also adding dependency to tie this to the bug that added the assertion, but I don't know for sure whether this actually is reproducible that far back [hence depends-on rather than regressed-by])
Bug 1919415 Comment 5 Edit History
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(also adding dependency to tie this to the bug that added the line of code with the assertion that's failing here. Note that I don't know for sure whether this actually is reproducible that far back [hence depends-on rather than regressed-by])