As far as I understand, the Qt5 stuff is currently only enabled for the Mac/OSX platforms because the corresponding launcher there had to be discontinued (well its dependencies got phased out by Apple) - and shouldn't be used anywhere else (yet).
So I believe this should be something else - and looking at the code in question, I think it's this:
https://gitorious.org/fg/flightgear/sou ... Dialog.cxxHowever, it is true that it's indeed Zakalawe who's been adding native platform GUI stuff (it all started out with the Mac menubar ...) - you can usually spot his work by looking for the whole "postinit()" logic all over the place...
But I guess, I'll leave it to windows folks to actually test this one and report back here. Keep in mind though, that Zakalawe is also primarily on Mac OSX, so Windows related commits may not have gotten enough testing (yet).
A possible workaround is discussed at SO:
http://stackoverflow.com/questions/2378 ... ot-showinghttps://www.opengl.org/discussion_board ... dialogs-pbSo if you can patch the C++ code accordingly, you should test this - otherwise, please file a proper bug report linking back to this posting/thread for additional pointers.