On gentoo linux (and probably on all linux systems) the compose key does not work in TeXstudio.
How to reproduce: Set compose key to RWin, start TeXstudio type RWin+" o
The user expects to get an ö now, but TeXstudio shows "o.
TeXstudio 2.5.2 (SVN 3651)
Using Qt Version 4.8.4, compiled with Qt 4.8.4 R