Here is a SWF demonstrating the problem.
2018-07-24_1017.swf (application/x-shockwave-flash), 1.23 MB, created by William Friedman on 2018-07-24 07:19:05 UTC
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Creator: William Friedman
Created: 2018-07-24 07:19:05 UTC
Size: 1.23 MB

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Attachments on bug 118910: 143724