This turned out to have been partly-caused by the patch that landed in bug 1923251. Masayuki and I backed that out earlier today, which will probably make this no-longer-reproducible in tomorrow's nightly, hopefully.
Bug 1926087 Comment 3 Edit History
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This turned out to have been partly-caused by the patches that landed in bug 1923251. Masayuki and I backed that out earlier today, which will probably make this no-longer-reproducible in tomorrow's nightly, hopefully.