How do I force line-breaks in my index?

Question

I have a big book with a lot of index entries. I'm getting some overprinting in the index, like this:

I've created a MWE:
documentclass[12pt]{article}
usepackage{makeidx}
makeindex
begin{document}
This is the first page.
index{European Organization for Nuclear Research (CERN)}
index{fable book}
index{fable forger}
index{fable-framing}
index{fable play}
index{fable teller}
index{fabrication tax}
index{fabric spring}
index{fabric tire}
index{face-about}
index{face-ache}
index{face ague}
index{face angle}
index{face-arbor}
index{face bath}
index{face-bedded}
index{face bone}
index{face brick}
index{face card}
index{face-centered}
index{face chuck}
index{face cleat}
index{face cog}
index{face cord}
index{face cover}
index{face cream}
index{faced-lined}
index{face dresser}
index{face dressing}
index{face enamel}
index{face gear}
index{face guard}
index{face hammer}
index{face-harden}
index{face joint}
index{face lathe}
index{face lifting}
index{face mask}
index{face massage}
index{face milling}
index{face mite}
index{face mold}
index{face molder}
index{face-off}
index{face-on}
index{face paint}
index{face painter}
index{face painting}
index{face par}
index{face pit}
index{faceplate coupling}
index{faceplate jaw}
index{face powder}
index{face presentation}
index{face rouge}
index{face-saving}
index{face side}
index{face spanner}
index{face specialist}
index{face sponge}
index{face stone}
index{face string}
index{face towel}
index{face urn}
index{face value}
fontsize{9}{10}selectfont
printindex
end{document}

Without the 12pt, the index looks like this:

With the 12pt, it looks like this:

I'm compiling this with:
$ latexmk  -pdfxe -bibtex idemo.tex ; open idemo.pdf

(I get the same results with -pdf instead of pdfxe)

egreg · Accepted Answer

I'd use imakeidx, which internally uses multicol instead of the standard two-column format. It also doesn't require a separate run of makeindex.
documentclass[12pt]{article}
usepackage{imakeidx}

makeindex
indexsetup{othercode=fontsize{9}{10}selectfont}

begin{document}
This is the first page.
index{European Organization for Nuclear Research (CERN)}
index{fable book}
[...other index entries omitted...]

printindex

end{document}

You can also consider three column format:
documentclass[12pt]{article}
usepackage{imakeidx}

makeindex[columns=3]
indexsetup{othercode=fontsize{9}{10}selectfont}

begin{document}
[...the rest is the same...]

David Carlisle · Answer

It's not really related to indexing or the document class option, just that some sequences of words are hard to break at some line widths.
Using either
fontsize{9}{10}selectfontraggedright

or
fontsize{9}{10}selectfontsloppy

allows it to break without being over full and sticking in to the next column, the effects are diffeent though you could choose which you prefer.

How do I force line-breaks in my index?

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