Frankly, I think it will be easier to do all the drawing in SDL and just have your editor work with two separate windows, the SDL-rendered one and the form-designer one. Just create a timer on the form to periodically check for SDL events and update the display surface if necessary, and that's pretty much all. You could, eventually, go with having all SDL-related stuff handled in a thread, but, in my own experience, that's too much of a complication in exchange for very little benefit.