Ok, I understand now what is the issue here. Thanks for your help. I also tested this on Mac OS X 10.14 and the issue is reproducible. The Release 65 version, Beta 66.0b14 and Nightly 67.0a1 (2018-03-18) are affected by this issue, I will update the tracking flags.
Bug 1057858 Comment 18 Edit History
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Ok, I understand now what is the issue here. Thanks for your help. I also tested this on Mac OS X 10.14 and the issue is reproducible. The Release 66 version, Beta 67.0b1and Nightly 68.0a1 (2018-03-18) are affected by this issue, I will update the tracking flags.