A SDL2 backend has been contributed to OpenTK here:
I've been using the SDL2 backend for around a week now and it's working quite well. There are a few kinks left to work out (a rare crash-on-exit bug and some oddness with multiple windows) but it's already more stable on Mac OS X than the Carbon backend ever was. Still need to test on Linux.
Tested on linux and it is working great! There is an issue with getting-setting the window position (different origins?!) but it is otherwise working as good as on the other platforms.
I will push out a new beta release shortly.
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