Both modern input method framework support this, including iBus and Fcitx.
If you cannot get it work, you might miss some package.
The problem for "pure" X11 application is historical, when XIM (X Input Method) is used, Compose from XLib will never work, but for such case, input method framework will take the response for Compose, so still no problem.
Posted Jun 28, 2012 3:49 UTC (Thu) by emk (guest, #1128)
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Actually, relatively few input methods handle XCompose in any sort of sane fashion, because not many users need to input both European languages and CJK languages. The released version of ibus is a hopeless mess, for example, though it's allegedly fixed in git with ibus-xkb. (Not that I can get any of it to build, thanks to random undeclared API dependencies.)
There are several major exceptions, including uim, which handles xcompose quite nicely.
GNOME and input method integration
Posted May 6, 2013 7:31 UTC (Mon) by gioele (subscriber, #61675)
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> Both modern input method framework support this, including iBus and Fcitx.