TGUI is a cross-platform modern c++ GUI library.
Although TGUI was created for SFML, since TGUI 0.9 you can use custom backends and an alternative backend is provided that uses SDL2 with modern OpenGL.
A .Net binding for the library is available at tgui.net (only for TGUI 0.8).
The gui is easy to use, with only a few lines you can e.g. have a fully functional TextBox on your screen. The widgets can be created by just using colors or by using images, making the look very customizable.
TGUI comes with its own Gui Builder, which allows designing your gui more easily. The widgets are loaded in your program by simple calling gui.loadWidgetsFromFile("form.txt").
TGUI will work on multiple platforms, it can be used on Windows, Linux, macOS, FreeBSD and experimentally on Raspberry Pi, Android and iOS.