See bug 1364714 comment 32: The height of the red background may show a slight (single-pixel) difference in height, depending on platform and font options; the freetype 2.8 update, which results in slightly different ascent/descent metrics for some fonts, caused this to show up on Android, but bug 1364714 comment 32 indicates it may also occur on desktop systems, depending on font size (and likely other factors). It's not immediately clear to me whether this is a test bug -- the test and reference files depending on an unsafe assumption about different document structures rendering the same -- or a code bug whereby we are computing a box size incorrectly (rounding error?) somewhere. (See also bug 1314684; this test was disabled some time ago on Windows, I think for a similar mismatch.)
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