Castle Game Engine features a number of user interface (2D) controls. You can build a complete user interface in the CGE editor, you can also build and modify the whole UI using our Pascal API. Our UI controls have parents, anchoring, UI scaling. If you set the anchors right, our UI will "just work" at various screen resolutions, aspect ratios and platforms, and can seamlessly adjust to any desktop or mobile screen.
As an example, take a look at this design that is used for our inspector (see the Inspector invoked by F8 movie). Coupled with the right code, this design implements a non-trivial UI with toggable panels, scrollable list box, minimal scrollable object inspector, custom-drawn profiler and more.