For developing an application optimized for LXDE, should you choose GTK2, GTK3 or QT?
Most LXDE apps are currently using GTK2, but it looks like GTK2 is becoming dead and the LXDE developers are porting everything over to QT
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As far as I have understood the situation with LXDE, they are not 100% sure to switch to Qt.
Since there will not be any further development on GTK+ 2, they have to choose between GTK+ 3 and Qt. It looks like there will be two version of LXDE one with Qt and the other with GTK+ 2 or 3, until they decided which to choose.
To answer your question, today you can't really tell in which direction it will go. My advice would be, it doesn't really matter which framework you will choose, since on most unix desktops there are all three frameworks installed.
I would not recommend GTK+ 2, because this is definitely not beeing continued. The best bet looks to be Qt, but it is a gamble.
Given it's developed using the GTK+ Toolkit I would assume that developing using GTK2 would make the most sense.
excerpt from LXDE wikipedia page
LXDE is written in the C programming language, using the GTK+ toolkit, and runs on Unix and other POSIX compliant platforms, such as Linux and BSD. GTK+ is commonly used in many Linux distributions and allows applications to run on different platforms.