[XeTeX] Line-breaking algorithms in XeTeX

[John Was]

Dear All

Since starting to use (plain) XeTeX I've noticed something strange with the paragraphing/line-breaking mechanism which has never happened during the ten years or so during which I have used traditional TeX.  It is cropping up in the fourth issue of a periodical that I have set with XeTeX, so I'm pretty sure that it's not a random fluke.

(1) I sometimes get an overfull rule (i.e. rectangular box) at the right-hand side which will disappear when I either (a) attach the word causing the problem to the next word with ~, forcing it over (I sometimes have to put the word in an \hbox{} as well); or (b) when I increase the line-count by giving \looseness1 for the paragraph.  In the past, plain TeX would always make such decisions for itself and never generate an overfull rule when it could find a way to justify the paragraph without doing so.  This happens most frequently in the reviews section of the periodical, where  \looseness is set to -1 by default to save as much space as possible:  but until I started to use XeTeX, it was always the case that if the paragraph could not lose a line, then the negative looseness was ignored and the paragraph was set successfully with normal looseness  (i.e. \looseness = 0).  It was never (I think) the case that a tight looseness which generated an overfull box would get through and need manual intervention from me.  So has something altered in the way XeTeX is handling the line-breaks, giving priority to the looseness command even at the expense of generating an overfull rule, and even when zero looseness would cause that error to disappear?

(2) This is even more puzzling (and more of an nuisance).  For the purpose of sending contributors proofs of their reviews I start each review on a new page so that they don't also receive the tops and tails of adjacent reviews, but while initially typesetting I have the reviews running on consecutively, as they will do in the final published version.  There is a switch at the end of each review which generates a \vfill \eject when \ifseparatereviews is true, otherwise it just produces a \vskip: there is no other difference.  Yet I sometimes get overfull rules showing up (at random points) when the reviews are separated out, even though the same paragraph typeset without error while the reviews were set to run on continuously.  The problem almost (but not entirely) disappears if I double the \hfuzz when the \ifseparatereviews switch is true, but that is no more than a quick fix to prevent authors receiving proofs with worrying blobs at the right-hand side.  This seems incomprehensible, but as it has happened with four out of four periodical issues I can't be imagining it - and the commands are precisely the same as the ones I used when the periodical was typeset using traditional plain TeX, with no new parameters such as alteration to \spaceskip or anything else that might cause this to happen.

(1) and (2) seem likely to be part of the same problem (though not necessarily so).  Any ideas, or at least insight into what XeTeX is doing that old plain TeX didn't?

Thanks


John

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