See this post in topic "Ubuntu Phone, Tablet Developer Preview Released" in Phoronix Forum :
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- X11 is NOT used. They set the QT_QPA_PLATFORM environment variable to "ubuntu", which causes Qt5 to load /usr/lib/arm-linux-gnueabihf/qt5/plugins/platforms/libqubuntu.so. That particular library is linked against libGLESv2.so.2 from libhybris. So, with that said:
* Only Qt5 apps will run (or anything else patched to use libhybris)
* This allows them to use Android graphics drivers without modification
* Desktop Ubuntu will never use this.
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So all the GUI softwares will be new, written in Qt5. No previous softwares in Gtk, WxWidgets, Qt4, etc.