A minor observation on both Linux and Windows after bug 550487 and bug 565209 fixes on multi-line to/cc headers, for which I don't have an explanation. I've set the value of mailnews.headers.show_n_lines_before_more to 3, and in most cases I actually get three lines of addresses for headers which are that long. In some cases, when opening a message the first time, especially after a fresh start of Thunderbird and when both to and cc headings are long, I either see a list go into the 4th line by one address, or the 3rd line appears to be cut off too early. Going to a different message and back, the lists are displayed correctly, filling up the three lines as desired. My guess is that there is still some issue with the clientWidth computation not reflecting the final style upon first opening, but only later returning the correct width and thus making the calculation accurate. These errors apparently are rather small, the single-line case isn't affected at all, and enough of these small errors need to accumulate in order to trigger this borderline case. I'm filing with "trivial" severity as the default use case doesn't seem to be affected and the impact is minimum for the n-line case. Eventually, this may be resolved WFM or as dupe of a XUL bug (or may become that XUL bug), but I couldn't find any matching Core bug.