Comment 1 for bug 862401

Iain: The Ubuntu Mono is built around a width of 500 f-units; because these glyphs are not yet in the font, they get substituted from other wider fonts. If they are substituted from say DejaVu Sans Mono, these would have a width of 600 f-units, causing them to flow into the next cell.

If used in a non-cell rendered (such as Thunderbird), they will instead cause all of the following position to be out-of-sync and misaligned.