Thanks for the bug report and the test case! This appears to be a regression from bug 1718709. Alastor, would you be able to take a look at this when you have time? Please adjust severity and priority as you see fit.
Bug 1729420 Comment 2 Edit History
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Thanks for the bug report and the test case! This appears to be a regression from bug 1718709 (as verified via mozregression). Alastor, would you be able to take a look at this when you have time? Please adjust severity and priority as you see fit.